Ok I have a square wave oscillator at about 1kHz, made from a 74HC14 inverter, with on-time and off-time pretty equal to each other. The wave at the inverter's input is about 1V peak-to-peak, oscillating between 2V and 3V as per the schmitt hysteresis. The output is 0V-5V, as a CMOS part operating on a 5V rail.

But I want to have a signal line that stretches the off-time to be significantly larger without influencing the on-time at all. So far I can think of using a FET or analogue switch (or part of a CD4007) to switch a resistor in parallel with the main timing resistor, with a bypass diode in antiparallel with the switch if there isn't a body-diode there already. But needing a new IC for this sounds like a pain (though i may be able to use the inverters of the 4007) and the 5V rail doesn't give me much headroom for turning on a normal 3-terminal MOSFET or a JFET. Pic related. I might get better switching if I put the NCH and PCH fets in this 4007 stage in parallel, i.e. connect pins 1-4, 2-5.

Trying to modify the capacitance to ground would be easier to do with a transistor due to the ground rail, but this seems impossible to use since any extra capacitance or resistance I add will be equally effect both the rising and falling time constants.