Would this be possible with a vactrol?

[drezdn]

Could you make a circuit using something like a vactrol where the resistance side would be set up so you could connect it to a jack meant for an expression pedal, with the vactrol serving the function of the expression pedal?

[Aleph Null]

I don't see why not. Most expression controls are expecting to see a change in resistance. That's what a vactrol does best. You may run into trouble with values: most vactrols have pretty wide min/max resistance ranges. I'm guessing most expression pedals don't offer 10M resistance. That's something you might have to address in the design.

[midwayfair]

Many expression pedals require a voltage divider, and there may be a requirement to actually hit ground. A vactrol is not good for any aspect of this. However, there's nothing stopping you from trying it in general.