Not sure transit is designed for the same space. E.g. there seems to be no schema, and the default JSON encoding isn't super readable either.

Protobufs can be encoded as JSON and as text, so there are some ways to address the readability I guess.

Transit is a really good solution.

As for Protobuf, I tried using it in a number of places, but found it to be very inflexible (schema!) and hard to debug in case of problems.