Hi Sascha, if compressed textures are not supported on the card you can decompress the images system side before you upload them. The algorithms and various code examples are available if I remember correctly.

I don't do this either at the moment as I'm GL3.2+ only and the only configurations I know of hardware/software that support GL3 that don't also have st3c are open source drivers on Linux (as the support is still an extension (not core) and patented etc so trouble for open source development)

It should be quite easy to decompress however, the algorithm must only take a few short steps in silicon to return decompressed values, that means it's probably not going to be anything beefy or stream based.