That's a tough sell. The new rules will let you upgrade Z with miles and $500. If they allow upgrades from Z with SWU's, they are leaving money on the table-- money they might get from the sale of a C or D fare, money from the $500, money from the reduction in liability on the balance sheet from the use of miles. It is the same reason that UA will not (my prediction) allow SWU's to be used on S/T/K/L fares.

With the sameday thing you are getting warmer-- if it can only be requested and confirmed at check-in, you might be on to something. But a whole subset of complicated new rules to give something extra to customers that has a possible negative effect on revenue doesn't seem like the way to go.