Is it possible to wire an electric radiant floor as a heat pump in my Nest thermostat?
Just remembered that I never circled back here. 

I was able to get this behavior by wiring the floor as "Stage 1" heating and the main furnace as "Stage 2". To do this, the white wire from the floor heat goes to "W1" and the white wire from the main furnace goes to "W2/Aux". My "common" floor (green in my case, but not well standardized) wire went to "C" (optional) and my green furnace fan wire went to "G". 

And the red wires? This was tricky. Normally the Rh and Rc terminals are short-circuited but the Nest manual says that the Nest does this automatically. However, all combinations of the furnace and floor red wires to either/both of Rh and/or Rc didn't work until I manually connected the two to a single red wire and connected that to Rh. 

Final wiring: 


Now, when the Nest needs to increase the temperature a few degrees it will only activate Stage 1 (the radiant floor), but when it needs to raise the temperature a lot (like in the morning or when it's very cold out) both Stage 1 (floor) and Stage 2 (furnace) activate.