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 apparent patent troll by the name of Doug Rogers has a patent called "High Compression Rate Texture Mapping". IMHO 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 guy might harass you for money.
He tried to legally threaten Valve (indirectly, perhaps accidentally) because they were using crunch%5B1%5D, 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 might try and muscle in).
 Doug has taken issue with this post, accusing me of libel, so here's the squish author's post on the subject for some factual balance:
I would link to Doug's blog where a lot of the discussion took place and where statements were made, but he took it down from the web shortly after promising to cooperate with open source projects.
Yes, I don't like it when patents step on the toes of game tech R&D. I was pretty shitty with you at the time, Doug. We're not used to patent folk on our lawns.
If there's a specific fact I've gotten wrong, then please correct me. My apologies for calling you an [expletive deleted] over actions which probably weren't intended to be malicious on your part, but were taken to be malicious by a large number of people.