I was always under the impression you could file a flight plan while airborne, are you saying (genuine question by the way) that you can't do it once you are in CAS?
Obviously you put your own safety first. If your VFR clearance puts you below minimum safe altitude and you have no choice to get out you're going to have to declare an emergency and keep flying at or above that altitude.
Don't go scud running into the ground. Pilots have been killed this way before.
The earlier you make known you can no longer maintain VFR the better.