I like bijanc's idea the most by far.
Just use those CGI lines like they use for the line of scrimmage and 1st down lines on televised football games. That way you don't have to even deal with running out onto the track to physically lay down strips of adhesive tape across the track or any of that.
And even better still, for the 400, you could have "moving" lines that were set to move at, let's say world record 43.18 pace (or Stadium Record pace, or 44.0 pace or whatever you wish), with a separate little digital line moving along in each lane, so you could see how far back (or in front, if they went out really fast) everyone was relative to the line, so that way you would easily be able to tell who was in the lead in the race and who was in what place in the race during the first few turns pretty easily just by looking at how far in front or behind or whatever they were to their wr-pace moving-lines in their respective lanes. This would take the guesswork out of figuring out who was leading in staggered 400m races.
That also gives an interesting idea for the 100m, of putting in a moving WR-line, or O.R.-line, or National Record-line (for U.S. Champs etc), or what have you, in, say, the 100m dash, so you could see whether guys like Bolt or Gay or Blake were threatening a WR, AS the race was unfolding, during big races. Just like how in swimming during the Olympics the frequently put up a moving-WR or moving-O.R. line in the pool if any of the swimmers are threatening the WR or O.R. during their swim. etc