Just about the right size , I dont now if this would be better or worse than the present controls . But I think it would be cool for road trips if your gp2x is sitting on a stand in front of you , Or to really make the tv out function useful .
Off the top of my head, with that being a USB joystick, I think it should work "out of the box".
That is, if you consider that the box requires a USB-host connection over the EXT port with custom hardware, as well as a custom kernel or compiled kernel modules loaded up for linux to talk to the device over USB.
The only issue I can see is that you'll end up with two joystick devices, and you might have to make a utility to link /dev/js0 or whatever to the correct device you want to use.
There's a nice standard for USB joystick devices (through the HID driver, I believe). Assuming that SDL leverages that (and why wouldn't it?) then SDL based programs should be able to search for attached joysticks pretty easily. This still all assumes that the gp2x is set up to properly recognize the new USB device and create all of the proper device nodes, etc, for programmers to get at (without having to poke at USB themselves).
The only issue I can see is that you'll end up with two joystick devices, and you might have to make a utility to link /dev/js0 or whatever to the correct device you want to use.