And I would already have one suggestion: would it be possible to add the -nogui or similar flags, so that Pd isn't visible when running?
you *can* start the sub-process with the -nogui flag.
strange, I tried both "pd~ start -nogui $1" and "pd~ start $1 -nogui" and neither worked. The first doesn't start Pd~, the other starts with gui.
But: pd~ works also on Max 6.