Can I increase the power rating of a film resistor if I increase the cooling?
For a 100’C rise per 1kW input you would need a CPU style heat sink with 0.1’C/W thermal resistance with size and force air of a super cooler heatsink.  These are typical much larger(10x)  than your allocated area and have >10m/s air velocity over a large number of fins. 


water cooling would be necessary to reduce the area to 1 cm^2 with a very efficient heat exchanger.