In 1 word (full story below) : yes it works ! should even be possible with checked luggages but I didn't take the risk.

/mode rant On
An upgradable ticket (I had some Qcredits to use) from CDG to HAN (around 600 EUR, economy discount booking into Q class) is far much cheaper than to BKK (economy discount booked into O and economy promo booked into T, both non upgradable fare classes and an economy value V class ticket was more than 850 EUR return, although I saw some N class ticket on ITA, probably bookable over the phone, for 750 EUR). Hey QR ! Why making these upgradable tickets to BKK so expensive when you sell economy discount below 500 EUR ?! It gives a poor value to the Qcredits you award to Gold and Platinum members. And why not respecting anymore the fare buckets allocated to economy discount (should be W,O,T fare buckets) and economy promo (should be Q,N,S) although the website still indicates you can't upgrade economy discount tickets (which I did with my Q class ticket) and you can upgrade economy promo tickets (which is not possible with a T class ticket). Marketing geniuses seem to have took over Frequent Flyer Program geniuses again...
/mode rant off

As I didn't have so much time to enjoy a stay in Vietnam along with my stay in Thailand (but I'm always ready to add more flights and lounges to my itinerary), I managed to buy a CDG-DOH-BKK/less than 24 hour layover/BKK-HAN-BKK/no more than 14 days stopover (fare rule limitation)/BKK-DOH-CDG on QR website after having checked options on ITA. The good point is that buying an immediate turnaround in HAN saves you more than 20 EUR in Vietnamese airport taxes on your ticket ;-)

I decided to have a long layover in BKK (between QR832 from DOH landing at 7pm and QR834 leaving to HAN at 2:25pm) as I was traveling with checked luggages and I thought the 1h15 turnaround in HAN would be too short to go through immigration, get the luggages, check the luggages (check-in counters are supposed to close 1 hour prior to the flight) and go back through immigration.

At CDG, the check-in agent agent offered me to check the luggage to BKK instead of HAN without prompting for it (for those not familiar with checked luggages rules, a bag tag can't have twice the same airport on it, meaning that CDG-DOH-BKK-HAN-BKK is not possible : the tag must be CDG-DOH-BKK-HAN and someone has to pick up the luggage in HAN and tag it to BKK, and I didn't know it was possible when I bought my ticket). I didn't know if it would have worked with a shorter connection in Bangkok, although fare rules doesn't seem to have any short check restriction aside of to DOH.

I got 4 boarding passes at CDG check-in (CDG-DOH DOH-BKK BKK-HAN HAN-BKK) but could not change my seat at that time to get a front row seat on BKK-HAN and HAN-BKK, as they said they had no control onto these flights.

I got airport upgrades using Qcredits/Qmiles for both CDG-DOH (at CDG check-in) then DOH-BKK (at DOH), although I had to wait until 1 hour prior to the DOH-BKK to get my upgrade confirmed as there was only 1 seat available in J and they kept it until the last hour, in case they had a customer willing to buy it. Fortunately, there was no buyer and I could enjoy Al Mourjan lounge during my wait, thanks to my CDG-DOH arriving boarding pass in J :-)

I had 2 excellent flights in J, as almost usual with QR, but still don't understand why they don't offer a cheese plate on all long haul flight (it seems to have nothing to do with the flight time as DOH-CDG QR39 leaving at 7:30am offers cheese plate but not DOH-BKK QR832 leaving at 8:15am). In that case my back-up option is a traditional Arabic breakfast, including feta cheese, with a couple of glasses of Saint Emilion of course ;-)

I arrived well in advance at BKK to have time to enjoy the CX lounge (where the 1st world major disappointment is the replacing of true champagne by, I agree not so bad, Chandon sparkling wine) and could select at the check-in counter a front row seat on my BKK-HAN flight but still not on the return leg HAN-BKK.

The BKK-HAN landed 10 minutes after schedule, which tightened my already short international transit time. Fortunately, after leaving the aircraft, I was welcomed by a lady with my name on a sign, who, after waiting other passengers (for delayed luggages and other reasons, but I was the only crazy foreigner making the turnaround of course ! ) asked me if I had any checked luggages (which makes me thinks it's possible to make that turnaround with checked luggages) and helped my to go through the health check point (I didn't know if I had to fill this health check paper as I was only transiting in Vietnam but fortunately I did) then to the transfer counter, where she asked me if I needed to go through immigration, and then we had to wait... wait... hum... wait for what ? After 20 minutes waiting there (I would have preferred they gave me a lounge voucher instead) she asked me if I already had my boarding, which I had, and then she asked me to go to the boarding gate.

I took the transit escalator which lead to the safety check before entering the departure airside. I hadn't any immigration stamp on my boarding pass (all international boarding passes in Vietnam are stamped by immigration) but the lady who checked it before the safety check not even raised an eyebrow and let me go through. Fortunately there was only a few people in front of me because when I arrived at the gate, half of the passengers had already boarded the plane. The gate agent took my boarding pass, asked me if I were coming from BKK, to which I answered yes with a big smile and she smiled too :-)

I didn't think I really had any occasion to request a front row seat for the HAN-BKK flight (and didn't thought anymore burning 8 more Qcredits for an upgrade) unless perturbing the local staff so I went it with my already selected back row (no seatback kickers) window seat.

I spent most of the return flight writing this story, expecting it might be helpful for less adventurous ticket price optimizers.

Back to BKK, I was a bit afraid of the immigration agent asking why I left Thailand and came back in the same day without having a stamp from another country in my passport but no question were asked :-)

In the end I spent a good day enjoying the CX lounge and flying my favorite airline but I would not recommend this trip to those who don't have a valid Vietnamese visa or who can't benefit from a free visa on arrival, as I'm not sure all the check in agents in BKK would let you fly to HAN even if you don't intend to enter Vietnam (just read that story from 2014 to evaluate your risks doing that).

Matt