In case anyone cares, I'll follow up my own post to say that it is possible, and I think even technically allowed. ^ I managed to upgrade both directions, with the following caveats:
I could only get the tickets issued at the airport (likely processed as an instant upgrade), although AsiaMiles service desk was happy to request them---and they cleared several days before departure in both directions.
I was charged 17.5K (the one-way price) in both instances. This caused me to have to fork over cash to "top up" my account at the airport on the return, as I did not have enough miles in my account to cover the extra cost.
I was told to call AsiaMiles after the completion of my trip, and I would be refunded the 5K miles to my account in a few days (but not the top-up purchase fee). We'll see if they show up.
So, the short answer is it can be done. The longer answer is it is a huge PITA---and probably not worth it for regional J. As my first CX experience, I must say I was disappointed.