This is surprisingly absent from the OSSC. It seems like virtually every HDMI console supports both, the only exceptions I can see is that the Wii U only does limited while the OSSC only does full. Many people have displays / setups with flaky range autodetect and chose to just go all-limited for maximum compatibility.
I’d assume scaling and offsetting the OSSC’s full range to limited would be relatively easy in terms of implementation effort and runtime cost and this feature would help integrating the OSSC in certain setups. An option for RGB range would be much appreciated.