I've done it a number of times.

http://www.flyertalk.com/forum/Forum50/HTML/011528.html

Sometimes they will accept tix without endorsement. I think your chances are always best at the gate where they are more likely to make a quick decision and not worry about splitting hairs over the rules.

Otherwise, you need to get the ticket endorsed which involves the issuing airline writing something on the ticket, which can be as simple as a scribbled rule number involved and the ticket agent's signature.