I HATE that "rear brake locked up" feeling, but at least I've remembered to keep the pressure on it the few times it's happened to me, thanks to posts like these.
So, what about front braking during all this? When my rear wheel locked up on me in AZ in Nov in the rain (on that fatboy I rented with a bad rear tire), I pumped my front brake (put as much pressure on it as I felt I could without it locking up and letting up on it when I felt unstable), but was too afraid of going down to really jam on it. The result was that I slid right through the intersection I was coming up on - but it was something of a calculated risk, since we were out in the middle of nowhere and I was "pretty sure" there wasn't going to be anyone coming through it to hit me.
Since I was at a relatively low speed by the time I got into the intersection, tho, I think it would have been safer to lay it down than either to get hit or slide into a vehicle.
I was lucky there wasn't any cross-traffic.