Full support for RSASSA-PSS (RSA signatures with PSS padding) has been added in Rebex Secure Mail 2017 R6.
Yes, we are already working on this, and with works on .NET Core edition now finished (to be released next week), adding RSASSA-PSS and RSAES-OAEP support to Rebex Secure Mail just became our top priority. Most of the infrastructure work is already finished, we just need to integrate it into our Cryptographic Message Syntax implementation and make sure it works properly. There will be a beta soon. Sorry for the delay!