I have discovered that if I start both ESBs with
run -b 0.0.0.0
my problem is solved.
OK, so my assumption about the default port binding being "0.0.0.0" was obviously wrong, unless something was modified :-)
Yep, the default binding for AS and the ESB server is always 127.0.0.1.
This is to prevent people inadvertently exposing the server to the net.