It would have to be done via MPI_Info arguments, and we never had a request to do so (and hence, don't define such an argument). It would be easy enough to do so (look in the ompi/mca/dpm/orte/dpm_orte.c code).
MPI implementations generally don't forcibly propagate envars because it is so hard to know which ones to handle - it is easy to propagate a system envar that causes bad things to happen on the remote end.
One thing you could do, of course, is add that envar to your default shell setup (.bashrc or whatever). This would set the variable by default on your remote locations (assuming you are using rsh/ssh for your launcher), and then any process you start would get it. However, that won't help if this is an envar intended only for the comm_spawned process.
I can add this capability to the OMPI trunk, and port it to the 1.7 release - but we don't go all the way back to the 1.4 series any more.