I think signal injection into a coax stub is ok to create a signal -but of course, no use for running a heater - and PWM's are fixed frequency - we need another trick.
BUT...you got me thinking about the problem of using a simple PWM to give a controllable mark/space ratio. Using a simple variable LC circuit and fast Mosfet switching it might be possible to clip off part of the output pulse from a PWM as it exceeds your chosen trigger point and divert it into a dummy load. Effectively this would enable very short pulses into the 'real' heater coil thus widening the space between pulses - but at the expense of what I think of as the 'skyscraper' shape of the 'Superwave'. But cheap and easy, as the Bishop said to the Actress.