I’ve played with this before using an ATTiny to control multiple expression inputs, sort of like an expression sequencer or multi-control from one expression pedal.
There are a couple of ways you can do this fairly simply using LDR’s to give a purely restive path for the device you want to control. The easiest is to use a single LDR in a setup that is much like the Morley volume pedals. You then control the LDR with PWM from the control board. You are going to need to apply a curve to the PWM to make up for the non-linear response of the LED/LDR but this is fairly easy to do by taking a few measurements on the resistance. This won’t work in all settings though.
The more flexible way is to use two LED/LDR combos to build a virtual pot. You then use two different PWM outputs to set the response curve you want on the virtual pot. It’s a bit of a pain, but perfectly doable in a DIY setting. This method will work with every expression input I’ve looked into.
I don’t think this would be all that useful though in a commercial setting because of the difficulty in mapping the responses of each individual pair of LDR’s. Electro-Harmonix had that expression pedal with no moving parts that would work with pretty much any expression input. It uses a different sort of method to provide what I think looks like a resistive path to the expression input. I haven’t tried to pull that circuit apart, but it might be worth looking in to how they did it if you are trying to build a more commercially viable product.