I handle travel arrangements (I'm not a TA) for 4 business travelers. Upon my suggestion, they have been successful at least 6 or 8 times doing what you describe. They have never had a full fare ticket when doing so, and in fact, most often their ticket was booked in the lowest fare class.

Only once were they refused when there was availability. The traveler was turned away by a terminal counter employee (told they wouldn't take it without paying $400+ fare difference), so upon my suggestion, he went directly to the gate (immediately after being turned away at the counter). The gate agent put his name on the standby list and he cleared standby and was issued a boarding pass, with no additional payment.

It is also important for me to say that in not a single instance has a ticket ever had to be endorsed by the ticketed carrier. In each instance, the traveler simply handed over his ticket to the agent who would then issue him a boarding pass. In our case, this has always been on the return portion of the trip and with a paper ticket.

As to QuietLion's comment, I'm not so sure about that. CO and HP are interlined with e-tickets (and possibly other carriers are as well), so I would suspect that it's possible that this scenario could work between those two carriers or any other two carriers similarly situated.