I've been trying to figure this one out for awhile… I was thinking that some combination of parameters that I could link to the expression pedal, but I haven't figured out how to do it in a usable foot-controlled manner, yet. Anybody got any ideas?
I do know that the TF is already capable of self-oscillation. You can do that by turning the feedback up over 100% (several types of delays go to 110%). Being able to control this with either a switch (as OP suggested) or an expression pedal would be great!! (I just don't know how to do it.)