Actually, I seem to be confusing this bug with another one - party members stop following you because ga_unfreeze_party doesn't fire properly at the end of a conversation that uses ga_freeze_party. 
After having a look at some of the OC conversations, it seems that the indestructible party member bug might be caused by the 'Multiplayer Cutscene' flag being set to TRUE. That suggests that it's a hardcoded failure to un-plot party members again when the conversation ends. However it should just be a matter of setting the plot flag to FALSE, as Tchos suggested.
It wouldn't be too difficult to write a script that checks all hostile creatures within a colossal radius of the player and un-plots them if necessary.
Would talking to a companion work? Providing that their conversations are cutscene-style.
Only if the companions in question are still in the party, and if the conversation is set to Multiplayer Cutscene. And only if the bug doesn't happen again. 