if it can be taken off of and put back on the ps3 without it becoming angry with you (PS3 "you wouldn't like me when i'm angry..." *HUGE ASPLOSIONZ!*), then it should be modable in the same way as the psx version, though knowing little of the format the psn file uses, it could have to have a checksum updated (if it uses one of course, and would be a simple way for them to prevent modding, but usually easy to overcome), or it could use some form of encryption that would need to be decrypted first. but if it is exactly like a psx iso, then it should be as easy as shake and... er... rip and inject.
but knowing sony, it is likely that it will not be a simple move to pc, mod, and reinsert procedure, there will likely be a catch to it, so it may be modable like the psx version, but you'd have security to sneak past first (oh noes! get teh cardboard box O_o). it may not even let you take it off the ps3 easily either, i never used psn on my ps3, so i wouldn't know what to expect.
