I think WingX lets you enter the pressure altitude so it can calculate the delta from GPS altitude, and display an estimated pressure altitude thereafter. We could do the same. But you would need to keep up with this as altitude and distance traveled can really change that delta. So you would still need to pay attention to your actual altimeter. About the only value I can think of is more accurate warnings for airspaces. And even then, that doesn't really save you much based on the effort of entering the data.
But maybe I'm misunderstanding something...?