Thai are using the 777-200ER between BRU and BKK.
I found a seatplan that states the same pitch (34") for both 51-59 and 31-46.
This one says the same:
The 777-200 however has a different layout:
In this report you notice that at least at the launching date they used HS-TJW which is a 777-200 ER.
I think and I hope seatguru is wrong. You wille have 34" pitch IMO.
Btw we are also flying with Thai to the region in November 2012.
Will be visiting Laos-VTE and returning from Cambodia-PNH.
We got the same remark from our travelagent about limited seating possibilities in W class.
The low price of the ticket (+/- 800 euro) makes up for relatively unfavourable seating.
Hopefully we get the same food as the other pax.
In 2011 we went to Myanmar with Malaysia Airlines from Paris CDG.
Their 777 has 2-5-2 layout (and 34" pitch) which is much better and more comfortable for couples than Thai's 3-3-3.
Malaysia Airlines in Brussels did our seating by phone and email without hassle or questions. We could change it during online check-in.
However no limitations to certain rows with MAS.