Assume a legacy Linux application listening on a UNIX domain socket
Turns out socat can be used to achieve this:
socat TCP-LISTEN:1234,reuseaddr,fork UNIX-CLIENT:/tmp/foo
And with a bit of added security:
socat TCP-LISTEN:1234,bind=127.0.0.1,reuseaddr,fork,su=nobody,range=127.0.0.0/8 UNIX-CLIENT:/tmp/foo
These examples have been tested and work as expected.