I know that this has nothing to do with Florine, But I read about a proposal to use liquid Oxygen as propellant in a Nuclear Thermal or Solar Thermal Rocket. The Only reason that this was proposed, is that Oxygen could in principle be extracted from Lunar Regolith or Rocks.
Another thought (This one mine) is the idea of using multiple propellants in a Nuclear Thermal Rocket. First a dense or plentiful propellant is used and then a switch is made to Hydrogen. This is the same Idea behind proposed Tri Propellant Chemical Rockets.