You certainly can do it on CO-- so I would assume you can on NW. It's actually quite easy on a CO ticket, you just call up with the locator and you OP# and pin and they are more than happy to take your miles (assuming, of course proper ticket class and availability.)

I've done it a bunch of times, and as an additional "plus" if the person in question is being waitlisted for the upgrade in question, they "take on" your Elite status for the purposes of clearing the waitlist.

Again, only speaking of my experience on CO.