I am not certain if it applies in Bangkok, but Raffles Hotels frequently offer 24 hour rates, as opposed to fixed check in/check out times. Its certainly worth giving their local property, the Nai Lert Park (former Swissôtel) a call to ask. I have never stayed there myself, and can't therefore specifically recommend it on any other basis though, although Raffles usually runs a good hotel.

I also wouldn't be surprised if many other hotels would accommodate a request for a 24 hour rate if you called directly and asked. The Bangkok hotel scene is exceptionally competitive, and my guess is that they will be very flexible.

By the way, I have generally had good experiences using the Amex Platinum rate -- which guarantees you a 4:00 check out -- and asking for a further late check out. They will usually let me check out between 6:00 to as late as 8:00 p.m.