Just an idea.... which unfortunately I don't have time to research for you. But just wondering if it could be worth it for you to do some research into whether a transfer route is possible via a hotel program. (UA -> hotel program -> Avios).

A possible source you could start from might be :
webflyer dot com. If you look under programs on that site, there is a mileage converter across programs. So you might be able to identify a suitable sequence of transfers that would eventually achieve what you need.

Note : please do us a favor. If you do find a way to do this, please do NOT post ultra-exact details on flyertalk. This will lead to an immediate closing of whatever loophole you find, and no one else will benefit. If a rush into relevant programs follows, overall rules on whichever program you find that does this, will be tightened by the program owners to the disadvantage of everyone. But I bet with a bit of diligent research, you've got chance of finding one.

Good luck!