This is a bad idea and I can imagine most of the best CX customers would be highly annoyed by it.
But if it does get implemented, there should at least be a component that favors higher MPO members. For example, make it only available to MPO GO or higher (of course, I'm a GO so I would say that). Or offer it to elites at a (reasonable) flat rate first, then open it to bidding by everyone else.
If it's wide open to any steerage passenger on the lowest fare, and they can bid their way up past the elites for a few bucks, that would be a recipe for revolt.