My suggestion is that right now, we go with what we know and have tested. Either:

1) Define radius as a function of airspeed and bank angle. The user controls airspeed, and the bank angle is either set by UAVO or hard wired to 15 degrees
2) Using a UAVO callback, saturate radius as a function of some arbitrary bank angle. My suggestion is 2/3 of the maximum bank angle. Then by default, 22 degrees will be the maximum bank angle (2/3 of 35 degrees) and we will never request more than 60 degrees of bank (which is low enough that the autopilot could conceivably maintain altitude, although I'm a little skeptical without more evidence).

I prefer option #2, since this doesn't require any more changes to the UAVO, and it integrates nicely with the path planner approach.