Ok, so you use them for rendering with a certain program that you still haven't stated... If that particular program doesn't include the codecs necessary to display them as thumbnails on Win 7 x64, then your only hope is to find an x64 dll/codec/utility like you already have done (txviewer) and try installing it in compatibility mode to see if it works. Try downloading that for Vista (or XP) x64 and install in compatibility mode. I doubt it will work, but it's worth a shot. Compatibility Mode
Codecs for different raw files on Win 7 x64 are hard to come by. For instance, there aren't any RAW codecs for x64 that allow RAW image thumbnailing in explorer, either. If nothing works, your only option may be to find a program that can convert them to jpegs so they can be thumbnailed in explorer.