I've been browsing the hack pgaes, trying the appz, on 1.5, and 2.0. And I think, that as the save game works in gta, you deeply respected developers, and hackers, could make a hacked game, which can run homebrew apps.
If it's true that the UMD is in ISO 9660 format, you could hack an UMD, and write something like PSP-OSS to it. It could be the best way to run homebrew on any firmware.
thanks for reading.
think about it.