Considering in real life people wrestle alligators (or was it crocodiles?) by successfully holding their mouths shut so they can't bite, it seems reasonable to at least prevent bite attacks. The breath weapon one is a bit trickier, I really am not sure if I'd allow that...
Right. If the relative size between the man & dragon were similar as between the aforementioned man vs. alligators, sure. Otherwise, no way.
Okay. I guess you don't allow people to seriously damage dragons with swords either, since such fantastically huge beasts covered with scales cannot have their internal organs reached by 4-foot pieces of metal.
The size difference is represented by modifiers to the grapple check.
Someone in the thread said you could but the wording on pin says you're not helpless. Also, when pinned does your effective dex drop to 0, thus granting a -5 to AC (assuming dex 10+)?
Okay. I guess you don't allow people to seriously damage dragons with swords either,
Oh, so we're talking about picking them up now. I thought we were talking about grappling.Umm, what?
I allow a man to possibly pick up a normal sized alligator. I don't allow the guy to pick up a huge dragon. Get it?
Oh, so we're talking about picking them up now. I thought we were talking about grappling.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.