Cartridge heater frequency response
Yes, you can use a DC source. Life might be reduced under some conditions but probably no ill effects.
The typical construction of a swaged cartridge heater is like this ) from here:

As you can see, the internal construction is a coil of heater wire wound on MgO core and swaged into a nickel-iron-chromium alloy sheath with MgO powder filling.
At 100kHz the inductance in open air (ie if the sheath was not there) would be substantial, however it is likely that that, with the sheath, eddy currents induced in the sheath will cause hot spots in the resistance wire near the terminations and the heater will fail prematurely (perhaps very quickly) as the watt density limits will (locally) be greatly exceeded.
There's little reason I can think of to feed HF AC to a heater. Usually we time-proportion mains power to them with cycle times measured in seconds.