I suppose one way to slow it down might be to use encrypted binary modules for anything they've added (most obviously the DRM to try and stop commercial 2x games running on the lookalike system), and put the file to decrypt it on a hidden bit of the nand (or ideally in a separate proprietory chip). That way they would be able to release the encrypted binary modules, and the Linux source-code, but it'd be hard to get the two to interact on anything but a GP2x.
Wouldn't stop the creation of a copycat system, of course, but it might slow it down... maybe.
Alternatively, they could say "sodit, we've created a new standard!" and try and make both units work with one another's stuff in the hope that more people will buy your software. If you're able to charge even small fee for sales - say 10% of profits - and actually have decent copy-protection, then you might even benefit. Unlikely, of course, seeing as most of the GP(H) pricing model comes from profit on the system rather than any commercial games, but hey.