There is one function in TBGL called "TBGL_MakeTexture", it takes as input RGBA, RGB, BGRA or BGR block of memory. If you could return this stream from your TBTIL, it can be integrated seamlessly and even stay garbage collected.
TBGL_MakeTexture allows making nearest, linear, mipmapped linear and also anisotropic filtering, so I would prefer to do it this way, if possible.
with thinBASIC 2.x series, TBGL will support JPG and PNG natively.
TB 2.x series will be Windows XP/Vista/7 only, so module can rely on fact GDIPlus is always present, so it can provide this feature easily.
I was thinking about it and in the end it should be possible to develop the functionality as you say.
It will be also possible to "garbage collect" it, which basically means the module will release the resource added by TBGL_CreateTextureFromHandle in the end.
The slot2handle and back functions seems good idea.