This is great, I had thought a lot about in game LFS TV, but always thinking about conecting as spectator to a "fake" relay server. This is much more straightforward, even if it brings its own shortcomings.
Voiceovers are not much of a problem, you can always set up a ventrilo, teamspeak or whatever server. If the delay for all clients ends up being more or less similar (say within 5 or 10 seconds) and the speakers are watching a delayed stream themselves, it should be mostly ok for general commentary.
Maybe you could even somehow skip enough time frames for the latecomers once they are already watching so they can catch up and every ends up pretty much in sync.
Another cool thing would be having the mpr client start LFS by himself and load the mpr file when it is ready, I think the latest LFS versions can do that.
And crazier stuff. If the relay client acts as a server itself, the main server could redirect new connections to willing clients, and you could have potentially thousands of people watching almost live races. And then here come the sponsors... mwahahahahahah