JOEL - Of course the best answer to your question is, don't be late for Sacrament meeting :-) I know that's easier said than done sometimes. I really can't find any formal statement from a church leader on this subject, except of course those who tell us how we should not be late in the first place.
But I have heard some teach how the entire sacrament is an ordinance that one should participate in from start to finish or it probably shouldn't be done at all. For example you would not walk into a temple endowment session half way through or baptize someone without saying the baptismal prayer. The same with the Sacrament ordinance. The two sacrament prayers are a little different, where in the first we are reminded to take upon ourselves the name of Christ and to keep the commandments, while in the second that reminder is not there. So if we miss the bread we have missed that part of the covenant we are suppose to make when we take the sacrament.
As far as what I do personally, If I am out in the hall but there for both sacrament prayers I will take the sacrament, but If I walk in late after the first sacrament prayer has already been given, I will not participate, especially if they are in the process of passing the water.
I really don't think that this is something that a General Authority would give specific directions on. We should not be commanded in all things; we can make our own decision on what we need at the time or feel is appropriate.