I have been doing some research myself... And essentially the only way I could figure would be to hack a USB Mass storage driver.. which would be a low level C driver for the PSP to emulate a mount point kinda like USBHOST0 like for USBHOST FS in IRSHELL........
Also you would need a fully compliant USB OTG Dual role USB-to -IDE Tranciever... similar to the AMS Venus GOGO.. But I know the Venus GOGO is specifically not compatible (ALI Chipset) Ali dropped development on the chipset and it can't be flashed as far as I know......
So essentially it's not impossible but at this point it would require funding and the resources to put a team of like 5 atrisan low level programmers with like 6-10 yrs of programming experience on it for a couple of months and thats just not possible at this point.
Got 50-75K? I'll see what I can do
Also a custom PCB would need to be designed and printed them stamped with the appropriate chips so th PSP wasn't the size of a PS2 just to make it work
Inside Sony info would expedite the process but Sony really made this one difficult intentionally..... I truly believe Sony intended at least at the time to release a hard drive option at some point but. It's really too far down the line. So if they haven't done anything about it they aren't going to until the PSP2 comes out in like 2012. At that point 100GB SSD hard drives will be like $75-$100 making it a primary option for the PSP2... I'll bet by that point they will be running 1024 bit dynamic encryption with mandatory internet access verification at least every 6 months for the PSP2 to continue to work.
-Yngmike