There could be although legal issues when compression already compressed image files (don't find the link, but I remember trouble with an existing patent), at least I would do some research in this field.
Some ex-nVidia employee slash patent troll by the name of Doug Rogers has a patent called "High Compression Rate Texture Mapping". It's a dubious patent of an obvious idea, with plenty of implementations that pre-date his patent, but he was the first-to-file, giving him trolling rights.
Basically, if you treat the DXT blocks as individual items, and then compress them via quantization etc, this [expletive deleted] might harass you.
He tried to sue Valve because they were using crunch, but there was such an uproar that he backed down and promised to leave open source implementations alone (basically, if you're making money from "his" idea, he'll try and steal some of it).