by Haring
You can't FORCE your opponent to hold a card.If he has 2 cards left, he can still play the scoring card in his last action round (and thereby emptying his hand).
Quagmire & Bear trap has specific rules so that even if you are still under Q/BT, you can play a scoring card.
Only 2 reasons I believe where you loose by holding a scoring card is A) by your own mistake. B) by the "ask not" event, where the US player disposes of more cards then remaining action rounds and drawing all scoring cards. Higly unlikely but possible.
In every other situation you still have an oppertunity to play the scoring card.
Oh, and as you yourself pointed out, the "may not be held" doesn't mean "gets to discard at the end of the turn". But means you have to play it.
Cheers, Haring