Law 23 (Dead ball) doesn't say anything about umpires being struck by balls, other than:
The ball becomes dead when [...] whether played or not it lodges in the clothing or equipment of a batsman or the clothing of an umpire.
Either umpire shall call and signal Dead ball when [...] a serious injury to a player or umpire occurs.
Law 37 (Obstructing the field) only applies to batsmen.
Law 32 (Caught)
A catch shall be considered to have been fairly made if, [...] a fielder catches the ball after it has touched an umpire, another fielder or the other batsman.
That sort of settles it for me. An umpire touching (but not pocketing nor being seriously injured by) the ball doesn't have any consequences. If the ball was on its way to the boundary and suddenly it isn't anymore due to the umpire touching it, then that's tough luck for the batting team.
This is corroborated by the audio commentary to some of the videos referenced below.
But not on the last ball, I presume.
- "Gary Baxter Cricket Umpire Hit"
- "Cricket Umpire Gets Hit- Ouch!!"
- "Ball hits the Leg Umpire.3gp"
- "Michael Hussey hits UMPIRE Billy Doctrove...2008.."
- "Amazing cricket umpire hit in the head by batsman!!!!!"
- "Umpire catches ball.....or tries to, PETER PARKER!!!!!" (This one actually makes it to the boundary.)
- "Misbah ul Haq Hit the Ball on Umpire's BUTT (Private Part of Body) an"
PS: I don't think a fielder may use an umpire to field the ball, e.g., by deliberately pushing him/her in the way of the ball. That seems to fall under Law 41 (The fielder):
"A fielder may field the ball with any part of his person, but if, while the ball is in play, he wilfully fields it otherwise, (a) the ball shall immediately become dead. and [penalty runs and other stipulations.]"
Possibly other laws are broken as well.