CO has been doing what the OP suggest via M-Ups.
CO Plat - and only CO Plat - get free advance confirmed domestic upgrades when they buy M class fares (or above). CO also was a very active in offering instant upgrades to Plats - trying to get them to pay. Some would pay to skip the upgrade queue; others would not and often fly economy.
What the new UA willl do is not yet known.