Using an unbuffered StreamCopyThread, though this does not solve the problem with outgoing either.
Using -tcp to connect over a local socket makes it possible to throttle both incoming and outgoing streams.
Apparently something about redirecting stdio of the child process just was not working correctly.
(Tried turning off buffering on System.{in,out} without success.)
[maven-release-plugin] prepare release mock-slave-1.1