I was surprised to see Arduino Due (an SAM3X8E-based chip) on your list of supported platforms, but not Raspberry Pi (any model/version), which is also based on SAM3X8E. On the "Future Hardware Platforms" wiki page (https://github.com/RIOT-OS/RIOT/wiki/Future-Hardware-Platforms) I see RPi mentioned with a bulleted list of items/todos/notes.
While I like the Due, the sheer power and RAM capacity of the RPi is enticing to me. I would have thought that making RIOT-OS compatible for Arduino Due would have crossed RPi off the list as well since they are both SAM3X8E-based.
What would I need to do to get RPi supported? What if I am interested in supporting other SAM3X8E boards as well?
I think you got something wrong: the Arduino-due is indeed based on the Atmel SAM3X8E which is based on an ARM Cortex-M3 with *~96kB* RAM and *~512kB* Flash. The RPi on the other hand has nothing at all to do with the Atmel SAM3X8E. Instead it is base on some Broadcom Arm-A11 SoC with (depending on the version) *~512MB* of RAM and some *Gigabyte* of memory... So they have nothing in common.
The nature of RIOT is, that it is focusing on very constrained micro-controller platforms, where the Arduino-due is already on the upper scale... So basically: "If your device runs Linux (like the RPi), then run Linux!". For everything that can't, run RIOT