Safely? not so sure
Yes, you could probably charge more 4.2 volts per cell without catching it on fire. But once you go much over that 4.2 the battery is damaged. It's not going to give you longer flight times if that's what you are after.
The longevity of any chemical cell depends upon following certain guidelines established by the laws of physics and particular cell chemistry . Break the law and best case the cell will die VERY young. Worst case you have a fire and or explosion. Proceed at your own risk!!!! More than one person has lost their car in a fire due to improperly charging a LiPo in the car when flying on another battery.
Lipo fires are real. We may get lulled into a sense of security because we have never had an incident. Be careful.A close friend was using a multiport charger, set his batts to charge and went off to dinner. Came back and the entire house smelled of toxic smoke, he charges in the basement, the AC kicked on and spread the fumes throughout the home. Insurance covered the cost of the hazmat cleanup but what a pain in the ass.
I've never had a charging fire but, I did have a battery puff on the charger and was very warm at our last swarm.
These two incidents have motivated me to move my charging setup out of doors. Even the garage present an unacceptable rish in my opinion.
We had a battery once that a cell went bad and somehow the other cells shot up to 5v! 5v per cell! Ended up throw it in a metal bin outside because the local battery store will not dispose of batteries they deem damaged (very smart of them)
OK- yeah, that would suck. I have recently picked up a deep cycle marine battery for charging at the field.
I still haven't had a fire, but I did have a lipo recently puff on the charger, just got lucky that it reached a full charge and stopped before it burst. It was very hot and very swollen. I've moved my charging out of the engine compartment and onto the tailgate.