Yep, if the GM allows it the way to do it is make suggestion OOC. Personally I treat playing Int 7 barbarian as a chance to chill out & not worry about riddles! OTOH if I had to make a rousing speech to my Horde before battle, even if my PC had zero Diplomacy or whatever I'd still do my best to do a good barbaric speech.
Edit: Whatever the stats say, fantasy heroes are cool - they should look cool in play, like Ron Edwards says in 'Sorcerer & Sword'. You can be cool in a Diplomacy-0 way: "I have no tongue for this, Crom, but..."