I'd love to have a mature Cap'n Proto binding for Ruby! Particularly one that works across CRuby and JRuby.
I was just talking to @kentonv about it, and this approach (MRI cext) seems reasonable for CRuby. I think the JRuby version should bind to capnproto-java, which is unfortunately much less mature than the C++ version and, AFAIK, doesn't support the RPC protocol.
Getting serialization working across both CRuby and JRuby would be a good start.
Have you thought about making a project mailing list?