No. In telco, the -48V is referenced against ground, like the physical ground. If you're isolated, you can do this. but they would still need to be referencing the 'ground' to something ... likely the negative side of the main battery pack.
The reason why -48V is used is because it is provided as a bias voltage to give wiring cathodic protection, to prevent corrosion of telecom infrastructure. If you used 48V, it would not work. You need a negative voltage referenced against ground.
Voltages are all relative. It's like saying 'How do you get a height difference of 10 feet by digging?'
Well, you dig and then label the initial level as +10 feet, and redefine the bottom of your hole to be ground.