Thanks for the reply!

It seems I've managed to "deal" with Thai booking engine. I was thinking... if system allows you to pick just lowest fare, and lowest fares (SAVER for example) are not unlimited... Then you need to make system think that there are no more SAVER fares available, at least temporary! Then I was searching for one way flights within my multi city route (LHR-HKT and BKK-LHR), in first case (LHR-HKT) there were 6 SAVER seats available, and in case with BKK-LHR there were 5 seats available. And I just simply blocked those seats - made test reservations (seat gets blocked after passenger info input, prior to purchase confirmation). When there was no more SAVER rate available for my flights, I used multi city tool and got flights with FLEXI SAVER rate, with ability to choose upper deck. So now I've purchased flights, and pre-selected 76 row seats at upper deck using mobile version of the site.

Still can't do anything about special meals - there is window to select it, for each flight, but no way to save selection (no button to save/confirm), and when I close that window, nothing is being saved. I guess I'll just email to Thai to have it changed.