I had a friend who managed to book the Discovery Airpass - they got all the segments needed in in L in May - but the Samui segments were tricky - also they cannot go back on themselves (correct me if wrong - cannot do BKK-LPQ-(BKK)-USM). You'll struggle to get L class before then on that route - no seats BKK-USM at all in Feb I believe.

However, just a thought, you can use the REP-USM flight as it's a single flight number and prices differently (even though you touch down in Bangkok).

So a feasable option, should you fancy squeezing in a day in Angkor Wat, and the rare miracle of availability existing means I think you could do:

BKK-LPQ-REP-USM-BKK on a 4 coupon airpass.

Please correct me if i'm wrong - and this may not be helpful if you cannot do Angkor!