Elbow him in the throat?
It's one thing to wake me up with his cock rubbing against my ass or whatever signals "I'm *soooo* hard and need a warm cozy for my cock", but sticking it in while I'm sleeping (i.e., unconscious) is a no-no.
I've been told it's not a cozy (the knitted thing you put over the tea kettle, to keep it warm). It's a snood. Knit one perl two makes it ribbed.
But yeah, after hearing about the lack of permission, I have to think it's not a good idea, outside of someone with whom you have already discussed this. If it was an accident, that is another thing, entirely but might be hard to prove.
If you woke up, on the receiving end of an erection; is he sleeping or faking/raping you?
If it was a girl I was really close with, I don't think I'd mind if she used me, but I also don't think I could sleep through it. If it was me, I'd say "Hey, wake up. I want to use your body...."
If she's fine with that, yay. If not, well, there's always the morning when we can both enjoy it together, conscious, happy and most importantly, consenting.