This isn't possible as the PSP was built as a client for USB, not as a host. Don't go away thinking that I'm wrong because the GPS and camera must be clients and the PSP must be the host; it's actually the other way around.

Simply screwing with a USB cable is not going to get you anywhere. The PSP's hardware was not built to act as a USB host - and is therefore essentially incapable of doing so. Without, that is, connecting both the PSP and the USB device to an 'adapter' which acts as a host to both - though you would need a huge amount of driver software. Don't believe me? Believe Fanjita.