I have a strong suspicion that it would have to be designed into the lens from the beginning. I believe adding one to the existing RF lenses wouldn't work if I understand your question correctly.An interesting comment, JohnC and leads one to wonder if a dedicated RF drop in filter adapter is possible, an option that would add increased flexibility to existing RF lenses, especially at the ultra wide end. Perhaps our fellow CR engineering wizards can chime in here, as to whether the flange distance requirement or other engineering restrictions would negate this possibility. I have a large box full of various screw on filters that I rarely use, ranging from 52-95mm, as well as various adapters to hold the 100mm square filters.
On another note, I do believe it would be beneficial to design ultra-wides (particularly those with convex front elements) with that capability. I suppose the use case isn't large enough overall to support that unfortunately.