While I agree that stronger is better (and thus someone founding a contracts bureau for this should use the latest/greatest hash), I'm not sure collisions are a

While I agree that stronger is better (and thus someone founding a contracts bureau for this should use the latest/greatest hash), I'm not sure collisions are a real risk here, since my understanding is that a collision is going to include a lot of junk to make the collision work. So any collision-based forgery is going to include large instances of complete gobbledegook so that it'll be really obvious in court that there was tampering.