Sorry for popping this old thread but just noticing it.
I feel like I should contribute here... I see that you've run into the libPNG problems. I cross-compiled ScummVM for the PSP a while ago, I even see that my "instructions" (which were more of a build process that I transferred over via e-mail or irc, I can't remember) made it into the [http://wiki.scummvm.org/index.php/Compiling_ScummVM/PlayStation_Portable], in the "Hard way" no less :S
Building for the PSP was not really straightforward back then as the toolchain was not continuously available, even now their webpage is down, though github is up. I'm not sure what the status is on their jim.
The trouble with libPNG was that even when including the missing defines of the newer version, it compiled correctly but did not run correctly, I can't quite remember what the issues were, but I remember running "Beneath a Steel Sky" with some messed up sprites. Not too much, but annoying.
I'm not sure how it will handle ResidualVM to be honest, as the port ran into some mutex and locking problems or races when it needed to load large stuff into memory. "The Dig" locked up in a specific place without fail, and so did Sam and Max IIRC.
(In hindsight those might have been engine issues and not port)
I think I still have my PSP2000 somewhere (if I haven't salvaged it for the display), so I might give building ResidualVM for it a try... No promises