I don't understand what you mean by your software is too slow to measure the echo pulse because the speed of sound is very slow compared to an MCU. Usually the problem is that the MCU is too fast and requires too large a counter register to easily measure something as slow as an ultrasonic echo. You could just tie several registers together. And every time a smaller register maxes out, it resets and increments the next largest register.
But the type of device you are describing...wouldn't it simply be a fixed frequency oscillator that is using the echo pulse as an enable/disable. An oscillator that has an integrated enable/disable can do this (obviously). But if said oscillator takes too long to start up after being enabled, then a regular (always running) oscillator can simply be fed into one input of an AND gate and the echo pulse fed into the other input of the AND gate. The result is that the AND gate passes the oscillator value whenever the echo pulse is HI, allowing it to serve as an enable.
EDIT: I just noticed you already described your device in terms of an oscillator using the pulse echo as an enable signal...well if the oscillator does not have an integrated enable, then using an AND gate like I described is the way to do it.