Electronically adjustable PWM with 555 timer
Linear's LTC6992 might be what you want.



Figure 1. The LTC6992 gives a PWM output at a set frequency up to 1 MHz for an analog input signal of 0 to 1 V.

You could then use your low frequency PWM and low-pass filter to generate an analog voltage, scale this down to 0 to 1 V and generate your high-frequency PWM with the LTC6992.

There will be a response time penalty with this approach and this can be calculated based on the RC time-constant of your filter.