Setting environment variables inside RMAN?

The only way I know to set environment variables is to do it before I actually call the RMAN executable.  I have a need to call the RMAN executable and THEN set my variables such as ORACLE_HOME, ORACLE_SID, etc. 
We're talking about unix by the way.  I know about the RMAN HOST command but that executes your commands in a subshell and env variables are never propagated to the parent shell so I don't think that will work.

Thanks in advance