I ended up flashing the LVY47H image, which is the version that ships on the Nexus 6 if you bought it directly from Project Fi, and I think that helped some of my issues. I finally did see it lock in to a Sprint LTE "geofence" after about 30+ minutes of it switching back and forth. But others may not be so lucky.
Anyway, after some digging around in the Project FI app (Tycho), I came across these phone dialer codes (aka "secret codes"), which can be used to influence which network Project Fi jumps to:
DISCLAIMER: Use these codes at your own risk! I don't know if there are any side effects to these. I am not responsible for any disruption if it any of these break your Project Fi connection!
These are standard dialer codes, so to use them you just open up your (stock) dialer app and type the code. You should not actually hit the call button as if you are actually going to dial the number!
- Force jump to Sprint:
34777 = FI SPR
- Force jump to T-Mobile:
34866 = FI TMO
- Force jump to the next carrier:
346398 = FI NEXT
- Re-enable automatic carrier switching:
342886 = FI AUTO
NOTE: I don't know for sure, but I think you might have to do this code to reactivate the automatic network switching after using one of the above to force it to jump to a different carrier!
- Attempt to perform an automated "repair":
34963 = FI XME (or if you prefer, FIX ME :P)
I'm not yet exactly sure what this does to repair anything, but it throws up a dialog that says "Updating..." and adds a notification to the shade that says "Configuring your cellular network...". Presumably this is similar to clearing data in the Project Fi app and re-activating.
- Get information about the current network:
344636 = FI INFO
The info in here isn't really all that useful. You can get better/more interesting info elsewhere.