Womp womp.
You can’t.
I got in some ears with some folks at Epicor about the necessity for this and they said “we’ll see what we can do”.
The painful workaround is to bind your combo to a dataview, then you create an event for that combo box and call the dataview-filter-set and filter it with that.
I think I had something around that showed that. Lemme see if I can dig it up.