I was actually chatting with a colleague several months ago about how hard it would be to hack up a block-level FAT32 interface *above* a traditional file system to solve exactly this problem. The conclusion of that conversation was that, yeah you could do it, but it would be exceptionally ugly.

At least according to Wikipedia, there are plenty of third-party implementations of MTP for Linux and Mac. The catch is that they're not just magically built in.