Since there is no non-PM way (that I know of) to write a message from one
job/session to another's $STDLIST (other than :TELL or :WARN) then the session
has to request the info (by posting a read somehow). (Michael has already
described a good method for doing so. The session posts the read and the job
supplies the data when the job chooses to).
If you can live with the session having to ask for the data whenever it wants
it, AND you are on at least MPE 6.0, there is a new parm to the SHOWVAR
command.
SHOWVAR ;job=#j12345 (SM cap required)