What is needed is that one or more knowledgable modders/level designers work independently or together to write draft documents for capability extensions, different from normal extensions in QuakeC because they define entire sets of capability, ensuring compatibility as a whole with things that demand this capability subset.
After a draft proposal is written, it can be refined with feedback from the community, then written up as a technical document in final form and listed on a website like quaddicted, and then as each engine adds support for the capability, they can be added to that page's list of supporting engines.
Then level designers and modders can simply state "this requires URRE_MAPLIMITS_V1 - see
www... for a list of engines that can run this."