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.
They're not always leaving money on the table. Next month I take my third long-haul international round trip of the year on another carrier, one with flat beds in business (unlike UA, which flies virtually all ex-SFO international flights on 777s with the old seating), on a Z fare. What they're doing is leaving money on someone else's table.
This also explains why I will never be GS. I can't get it through work since my company isn't big enough to have UA throw boxes of GS cards at it, and I sure am not going to suffer through lousy seats and worse service on full fare to get there.