This is one of the approaches im considering for my own, Roder.
The approach mostly being communicating over json rpc which has become the standard for MCP so it makes it more approachable to agent developers.
Obviously its very much NOT mcp, its a low level events based rpc system for registering capabilities and extending low level primitives of the agnet itself not the model
I'm actually very close to being ready to release exactly that also in rust. I completely agree with your statement, extensibility is the most importnat feature.