If you start with something like the Fuzzy Fox, that has a switchable octave, maybe you can replace the switch with a pot to mix in the octave effect rather than turn it on/off.
There are many resources out there documenting how to insert a TRS jack in between the board and any potentiometer (the electric Druid filter fx build doc comes to mind). So boards like the parenthesis or any with a blend function could be converted.
You could also wire up a stomper to select between two pots; one of the pots being from the expression pedal — then you'd have a pre-set as well as go psychoschematicinsane!
Or if you just need two settings, just the switch and two regular pots or set the second pot as a trimmer that's preset...
If you start with something like the Fuzzy Fox, that has a switchable octave, maybe you can replace the switch with a pot to mix in the octave effect rather than turn it on/off.
This is of course not for a switch, but good general instruction. All credit to electric Druid for the info. I think replacing a switch would require including a blend circuit and not just an expression jack/pedal.