Again the psp's usb port will not function as a host, you will not be able to directly plug ANY usb mass storage device into the usb port and have it work unless it can function in host mode. They used some kind of intermediate board to bridge 2 slave devices together then used IRshell and the USBhost function. As stated in the article...
"The internal USB mass storage mod packs a flash drive inside the PSP, and includes a PCB board that allows the drive to act as the USB host, since the PSP only works in USB slave mode"
If you could build a board or find a schematic for one, you could possibly do it. It sounds like a lot more work than its worth, but i think the hardest part would likely be the firmware to drive such board.
I have seen a couple other options, but both use the memory card slot. Both decent devices in my opinion. You could get one of these and max out the abilities of the fat32 file system.
Or i also saw this, which is an intriguing device that used to be offered by Lik-Sang.
I dont have a clue where you could find the Lik-Sang device, but the other is offered on ebay and you can have up to 32gb of storage if im not mistaken.