sure
not exactly the best way to get a router
but if you goal is tinkering, then why not
you would need some carefully selected (open ppc driver) but still of the shelf hardware (1-2 usb lan, maybe a usb wlan)
if going the homebrew route you would probably need to write the software from scratch, I don't think anyone has ever done something alike
or you could use linux which should already have anything needed