Nope. RF is too shallow. Thats why they had to redesign the ND mechanism for the C70, to be able to fit it all in.I guess the question now: is it possible to upgrade the C300 Mark III and C500 Mark II to the RF-mount (especially useful for the former with the EF-EOS R 0.71x Mount Adapter) or they weren't made to be compatible in the first place, and they are going with either of these new models to push customers towards 8K.
And of course they will also offer the smaller C50, C70 and C90 with the older sensors for those who don't need 8K or don't want to spend more. 6 cinema cameras with the RF-mount (plus the R5 C) are probably more than enough.