Some discussion came up on the mailing list about adding Level 3 RPC support:

https://groups.google.com/forum/#!topic/capnproto/iXuK8wjmG8o

Louis was looking to arrange a video call around this; @zombiezen I know you don't have a ton of bandwidth for this project these days, but thought I'd ping to see if you'd be interested in joining us? No pressure.

I don't know Louis's GitHub handle, but I'll cross-link this issue on the mailing list.