CFR SCCA did a couple of events like that last summer. We'd run about a half-dozen cars and hold them at the far end, bring back the group, run another half-dozen, bring 'em back, and so on. It was certainly nice having the whole width of the runway to work with, and there was certainly no problem with multiple cars on course! The event flow wasn't as quick, of course, because of the pauses necessary to bring cars back to the start end.