No, I don't believe the "natural 20" / "natural 1" rules apply to a grapple-- it could be house-ruled though, I guess-- but otherwise it's gonna VERY HARD to escape from underneath a dragon who has a +58 to his grapple check. As well it should be! Your PC would need a +40 grapple check at least and have to roll a 20 when the dragon rolls a 1.
Again though, against that large of a party, it could be a good idea for the dragon if he can afford to take the -20-- that would give the dragon a +38, which still would be very hard to overcome. The dragon wins this check with it's first attack, then can dole out it's others against whoever it wants-- even using it's natural attacks to hasten the death of those crushed underneath it. Not a good day for those folks.
There are a few magical ways to escape though, I believe dimension door would be an option, and if I were a cleric you can bet I'd have a word of recall memorized. Also, there are magic items, one in particular I believe from the MIC called the Anklet of Translocation, I think... Also, don't discount freedom of movement or the Travel domain...
