This is a bit old, but there is a lot of PS2 homebrew that can run on the PS3.
For example: http://youtube.com/watch?v=LUfE84C8-2Q
The person who made that video has made a couple of other videos showing homebrew using swap magic discs and loading elf files.
I myself have gotten have gotten PS2 homebrew running on the PS3 such as the popular uLaunchelf file manager. Although you need to obtain a certain irx file to get USB support.
If you can get usb support you can easily copy files back and forth between the usb device and the virtual memory cards.
One of the best parts about this is that you can get save files from your original ps2 memory card, put them on the usb device and copy them to your ps2 virtual memory card. This does not work for PS1 saves because the PS3 is running in PS2 mode and allows you to assign only PS2 virtual memory cards to the slots
Also, the NeoCD emulator works great on the ps3, but it has no support for the mp3 files from the usb device. This means that there will only be sound effects and no music.
Maybe if a coder were to pick up open source homebrews such as NeoCD, they can add full support for the ps3.
By the way, Metal Slug on the PS3 rox, especially co-op!