This has also been referred to as a "manual sell" for award tickets and it was something PMUA agents could/would do for quite a long time up until a couple years ago when it was expressly forbidden by the airline. Manual or Long selling ran counter to UA's policy of Starnet blocking so agents were told not to do it and risked their jobs if they did. I expect that policy is still in force and you won't be able to find anyone to do it for you.