Need to serialize message as protobuf and send it thru Solace appliance.
I don't know of a specific example with C, but @TomF wrote this blog a handful of years back that covers the generic steps you'd take to use protobuf (or other 3rd party data serialization systems) with Solace messages.
Can you take a look and see if that gives you what you need? To simplify (maybe overly) do what you want using protobuf, serialize and stick it in the Solace message as a binary attachment. On the other side you would then grab the binary attachment, deserialize and you've got your object back.
By the way, we'd love it if you could share your code as an example of how to do this with C and protobuf when you figure it out