I didn’t dare touch the brakes (very inefficient drum types anyway) and when I did start slowing down I got off at the next exit and stopped to recover from the shock and near death experience!
I know what you mean, and you probably did the right thing: Even tapping the brakes in that situation in something so unstable may not have been the wisest thing to do.
For what it's worth I had a similar experience once, when a tyre blew on my racing bike going down a very steep and long hill. I was going full bore when it went
blam! A lot of bucking, shaking and white knuckles followed for what seemed like an eternety, but after a while the hill ended, and I got it stopped. Then I did exactly what you did. Sat there for a while recovering from the shock, before I got around to changing the tyre and be on my way. I had no speedo on that bike but some other cyclists in the same race did, and some of them claimed to have done over 50mph down that hill...
/Claes