Sorry, just found out about the "--debug-daemons" option, which
allowed me to google a meaningful error message and find the solution
in the archives of this list.
For the record, the problem was that the "orted" being launched on the
remote node is the one from the system-wide MPI install, not the one
in my home dir.
It seems that "-x PATH" does not affect the search for "orted"; would
it make sense that "-x FOO" adds also a " -o SendEnv=FOO" in the "ssh
remote-node orted" invocation?