I'm not sure I fully understand what is desired here, but as far as forwarding TCP requests, I was able to do this using:
TCP In (stream of buffers) -> TCP request to Modbus server (return after timeout) -> TCP out (Reply to TCP)
In my setup, I had Node-RED listen (the TCP In) on port 7000, and the TCP request went to a local Modbus server running on port 7001. You'd then have to decode the responses from the TCP request node.
You could also decompose the incoming request, compose a new request with a Modbus node, and then compose the response needed for the TCP out. Doesn't really look like there's an elegant solution here without getting dirty.