So this is my first post here.
After about 1 month of work, the experimental SSIM RD tune has been added to the mainline branch.
We've already experimented on my fork of the branch, and have seen good results over the other tunes in the encoder:
https://github.com/BlueSwordM/SVT-AV1
The results are decent.
I've personally tested it on my "psy" fork of SVT-AV1, and it usually looks a bit better than the VQ tune, and it is somewhat more competent at higher fidelity levels.