Yes, I have been very involved in the Solid authentication panel. From the perspective of Trellis, it's principally a matter of supporting JWT-based access tokens, which Trellis already supports. What the Solid-WebID spec adds to this is DPoP (Demonstration of Proof of Possession at the Application layer: https://tools.ietf.org/html/draft-fett-oauth-dpop-04
), which is really just about adding a small JAX-RS filter. The idea is that (eventually) support for DPoP validation will be integrated directly into some upstream libraries: Smallrye and Quarkus: https://github.com/smallrye/smallrye-jwt/pull/277
The other part that is added (from the perspective of the resource server, i.e. Trellis) is WebID validation, which is also just a simple JAX-RS filter. Those two parts (DPoP and WebID validation) are not currently part of Trellis, but it would not be difficult to add support for them.
With KeyCloak, my experience has been that it works pretty well in simple, single-node arrangements. But once you try to use it in an HA/Clustered context, it gets really squirrly.