Personally, I'm waiting for this to happen. I've been putting it in the game suggestions thread for two years now.
I'm a firm believer that it would be a great idea, but at the same time, a complete and total nightmare, with highly entertaining consequences. I'd imagine that the way it would work would be a relatively simple challenge, that would only be scored if it happened during the autonomous period. That way, teams could have relatively simple dead reckoning programs, that they could line up with. However, you'd have to jockey for that lined up position, and if you are prevented from lining up (or you just plain run out of time), then highly entertaining antics could ensue. It would probably be a complete nightmare for teams, but great for the audience.
It's worth noting however, 30 seconds is way too long, I'd do 15 if the task involved moving significant distances and was hard to line up, 10 if it was easier.
I'm gonna do what Billfred did to show you what I mean:
Stack Attack: Well, if you don't know how this game was played, essentially, both alliances fought to end up on one slippery platform in the center of the field (there were ramps leading up to it on both sides) at the end of the match. Just lining up your robot and ramming up and over it would be highly entertaining to the audience seeing as it has a couple of good options:
A) Your robot gets air from moving to far, and missing an opponent.
B) You have a spectacular collision with another robot and one or both you go careening off the other side.
FIRST Frenzy: Raising the Bar: If you made only autonomous hangs count, then it'd be pretty boring if teams got lined up right. If they came from the floor everyone (with the notable exception of 190) would've fallen over.
Triple Play: Triple Plays autonomous would've been really hard to make autonomous. Pretty boring I think.
Aim High: Well, this would alter the game significantly, but autonomous ramping would've been fun to watch.
Rack 'N' Roll: Oh my. The GDC could've made this one easy (aka, ramps can only be lifted in autonomous) or impossible (only robots lifted in autonomous count, robots can not go behind the line until autonomous starts)
I think though, if the game is designed with the end of match autonomous mode in mind, it would be very sweet.