They didn’t put a drop-in slot in the 100-300/2.8. Highly unlikely there would be one in an RF version. It’ll probably have a gel slot like the EF 11-24, and Fotodiox might make a holder for their dinner plate-sized filters that fit the 11-24.
I have the EF 11-24, but not the 186mm front filters. The EF-RF drop-in is great for that, and because of that an RF version will need to offer more than 1-2mm on the wide end. I do have the salad plate-sized (145mm) front filters for my TS-E 17 and I’m very glad I no longer need them. For the same reason, an RF TS UWA will need something more. AF won’t do it for me, no need for my TS-E use. A TS 14mm would be tempting, but I’d probably add it not swap the TS-E 17.