idk, I didn't get far into PReps before deciding it was no-go for my intents.
That being, i was approaching it from the opposite direction: I noticed only that a Companion would not attack a creature that was PR-hostile to my PC (iirc, the creature was still blue to the Companion, while red to my PC -- this was well over a year ago, but I seem to recall being miffed that Companion couldn't even cast a spell to start his/her own aggro state). However, i did not check if the creature would initiate hostilities on the Companion ....
... I just have to get NPCs to ALSO set temporary hostile to companions/summons.
that would be nice. i mean it should work ...
Since some of the faction-functions are said to rely on PReps, including SetIsTemporary*(), I ended up going global and writing my own functions for setting and releasing reputations -- basically by plopping down a factionPig, setting its rep to the aggro'd party, then retrieving that rep after a certain amount of time (along with other conditions, like "no enemy faction member is in sight"), and deleting the pig ( PC faction doesn't do that, ofc ... how it feels about another 'faction' is decided by the player )
but again, that is global and I didn't even try to focus on a single player/PC.