If you sign up to an ANA account, then it is possible to search for award availability using the 'ANA Partner' link and select Thai Airways International.

A bit of a long winded process (but not too bad), and there is no guarantee that what Thai have released to ANA is ALL the award stock there might be (ie if ANA is showing no award availability, Thai might have some), but it has worked for me - and there is no other way that I know of other than ring THAI (which works well once you get a person...)