Then add a Custom tag to the copy of Martial Weapon Proficiency feat bootstrapped to the deity to mark it.
In an eval script on the deity, use findchild to get to that specific copy, and pull the WepProf tag from the feats selection to the deity pick. Something like this (assuming the marker tag is "Custom.DeiFavWeapChoice").
First 400
Code:
~Set the focus to the feat we marked
perform hero.findchild[BaseFeat,"Custom.DeiFavWeapChoice"].setfocus
~Stop if we have no focus, because somehow the feat we bootstrap isn't here
doneif (state.isfocus = 0)
~Stop if the focus has not yet made a choice
doneif (focus.field[usrChosen1].ischosen = 0)
~Pull the Weapon Proficiency tag from the focus to ourselves, the deity pick. This is what others look for when determining our favored weapon.
perform focus.field[usrChosen1].chosen.pulltags[WepProf.?]
This code is not tested, tweak as necessary.