Rockbox was never written in any part from ipodlinux or podzilla. We *did* take the hard work that the iPL guys had done on figuring out the hardware and use it to write the iPod port - but that's it - there's no shared code.
As to external accessories, it all depends on a number of things :
a) If you have a 1st, 2nd, or 3rd Gen Ipod. This is the only set of iPods that we know the serial hardware for.
b) Someone writing a serial driver using that information
c) Someone then writing an Apple Accessories Protocol to use that serial driver.
d) Someone then writing plugins(?) for Rockbox to drive particular accessories.
So far no-one has done b, c, or d.