Thanks for the suggestions. It turns out anything but localhost works, either :
ns_param datasource servername:55432:acsdb
or:
ns_param datasource 127.0.0.1:55432:acsdb
even:
ns_param datasource localhost.localdomain:55432:acsdb
Presumably using the real IP rather than 127.0.0.1 would work too but I didn't see any point in testing it. Also, it didn't require the FQDN, though that would surely work as well.
This seems like a bug in the postgres driver, since psql -h localhost -p 55432 treats the connection as a TCP socket. For consistency's sake I'd expect to be able to set up the server exactly the same way, not localhost=Unix Socket but localhost.localdomain=TCP socket.
