I'm sure it would depend on the attendant, but I might have to try that next time ;)

When I travelled HKG-BKK-DXB around easter, they already asked for shower assignments right after boarding in HKG, with 10/14 suites occupied. However, as the FA knew we were bound to DXB, she suggested we take our shower right after departure in BKK (we had been running around HKG all day and the lounge showers which are built into the toilet cubicles in HKG aren't really top-notch). This gave us plenty of time to have our meal and pay a visit to the bar. Time is quite sparse if you want to shower AND dine on this short hop which will hardly give you more than 2 hours of airtime. I am not sure whether anyone had their shower between HKG and BKK, most people seemed to have made an appointment just before landing.

In my experience, it is usually not a problem to get a shower assignment even if the F cabin is 14/14 (had that twice on my DXB-PEK-DXB legs); I'd asked the FA whether to dine first and then shower or vice-versa, she told me that this would be entirely up to me. By the way: Ask for the port side shower, there's much more bench-space to put your things onto.