I've been thinking about how to make a trivial casino contract (entropy has other uses as well, e.g. flip a coin to determine who in the marriage contract gets the car) in lieu of entropy, and came up with this (2 block process):
1st block: Make a commitment.
2nd block: You reveal the committed value, which is xor'd with every other committed value to produce a "random" value.
It's imperfect and can be gamed, but ought to work for most situations where at least two parties are not cooperating. Even then, however, it's not true entropy, it's more like a game of paper scissors rock.
However, just like paper scissors rock, the entropy of the resulting random value will be at least the maximum entropy of all of it's inputs. i.e. we only need one person to be playing randomly for it to work.