This gist describes how to run the proof of concept for protobufs-over-HTTP for the Pact framework.
It's worth noting that this work isn't yet in a production-ready state, and at this stage is a first illustrative step of what can be done in this space.
The ideas implemented here, are described in this presentation.
The pact-serialization-proxy component referred to in the presentation has a repo which can be found here.