A digital potentiometer uses a digital signal like a I2C or SPI bus to set its value. There is also a kind that has "step up" and "step down" signals. If you seriously want to do this without a microcontroller, then you'd have to build something like a phase-locked loop that ran on the current value of the potentiometer, and gated a high-frequency clock to generate step-up/down signals based on the phase difference between the incoming PWM and your PLL.
DON'T DO THAT!
It sounds like the problem you really have is this:
1) In comes a hobby servo PWM.
2) You want to control another PWM-controlled entity with a PWM that's some function of 1) but not a clone.
That could be done in an hour using an ATTiny85, which is a 8-pin AVR microcontroller for about a buck. Yes, it's the same size as a single 555 circuit, but it's a lot more powerful! It doesn't even need any clocks; it can run at 1 MHz or 8 MHz on a built-in resonator. I recommend using the 8 MHz clock.
Get a $20 AVR programmer, get yourself setup with avr-gcc and avr-libc, and write a small program for it; it would be a lot easier than trying to do this in discrete electronics. The program would do something like:
- poll for the incoming PWM to go high
- measure the duration of the incoming PWM pulse (use a high-frequency high-resolution timer -- I think there's a 16-bit timer in the tiny85)
- run whatever function you want on the PWM pulse width -- stretch it, invert it, scale it, whatever
- generate the output pulse you need by setting output pin high, waiting for the appropriate amount of time (timer again) and setting the output pin low
- go back to beginning
The delay between input and output would be the width of the incoming pulse plus a few microseconds for whatever math you're doing. The code could run entirely in polled mode; no interrupts or anything fancy needed.
