Right, 'The Tingler' already metioned this briefly, but I'm pretty certain that the cannoneer in the "Bring Fiery Death" sub-quests is indeed Kyle - "KG"/"Kage" - Gass; Jack's partner-in-crime in Tenacious D
The Monument Viewers have a nod toward both Iron Maiden's Eddie mascot with the screwed on cranium, and The Scorpions' Blackout album cover with the forks over the eyes
And finally the Fire Barons bear a striking resemblance to Rob Halford
I'm sure I've come across others but I can't bring them to mind right now.
Oh, and I feel sure that the hunter whose kill records you have to beat must be someone but I've no idea who. Why they didn't use Ted Nugent is quite beyond me.