Good to know, AC*SE.
I was recently on a full fare UA ticket JFK-SFO. They thought the upgrade chances were good, and they sold me 4 certs for $125. I ended up missing the upgrade by one person (I was next on the list).
I now have the 4 certs, so I hope to be able to use them on a discount fare next time I'm traveling. I don't think the system will allow an advance request, because it will probably reject a non-Premier UA member. I'm hoping they'll allow it at the airport.