I'm playing A Knife in the Dark, at 37 threat. Bill Ferny has 38 engagement cost and is still in the staging area. I want to get him out before next round, but can't optionally engage him. So, I play ally Elrond, discard the condition attachment "Light of Valinor", exhaust Glorfindel to quest and thereby raise my threat by one to 38. This enables me to engage Bill Ferny this round.
There are plenty of player cards with the "condition" trait, but I've never seen anyone use a player effect to discard them. I think this feels legal, though.