A few quick questions: What distro are you running? After modifying "joydev.c" did you recompile kernel modules? Did it give you any errors? Could you "modprobe joydev" afterwards?
I know nothing about the linux version but in windoze D3 you can assign axis to sliding -- you would use "Slide Vertical" and "Slide Horizontal" to do that instead of "Slide Up/Down/Left/Right". JIC..