I think it could be done, if the game was designed to support controllers that were connected to the USB ports. The vast majority of PS2 games don't utilize the USB ports at all. The games that do support the USB ports use them for various functions, such as keyboard, mouse, steering wheel, microphone, modem, or Ethernet adapter. Support for these peripherals has to be programmed into the game.
No game that I know of supports joypad controllers connected to the USB port. So, as it stands now, there probably aren't any games for which you could connect a joypad to the USB port and expect it to work in the game.
"PSP will elevate portable entertainment out of the handheld gaming ghetto." -- Kaz Hirai
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