Preface: I have 250H+ TT and study for my ATPL exams at the moment.
syseng68,
I can tell you from my limited experience that it would not be very useful. The relationship between TAS and GPS GS would be the wind, and then between IAS and TAS temperature, ambient pressure, altitude and humidity.
The only way to know the wind vector is to compare GPS vector with airspeed and heading information. If you have lost airspeed, you have lost all information about winds.
Winds do change, especially around convective activity. If the margin between stall and overspeed is small, GPS groundspeed is just about useless.
In my opinion, using optics or engine pressure readings sounds more promising as an ASI backup, because it tries to *measure* airspeed rather than deduce it.