I don't do squeeze box, so help me. How is this effect different from tremolo?
In the world of tremolo, we have the LFO, which is commonly a sine wave, or even a triangle wave, but for attack you could easily change that to a sawtooth wave.
Are you a stroller? Or do you stand fairly still in front of a microphone? Myself, I'd much prefer a simple foot switch, like a keyboard sustain pedal over som,e RF mercury switch thingie.
My experience is mostly limited to the Iorio Accorgan, I had a couple customers with those, and I saw one Cordovox years ago. The generator cab was as large as the amplifier cab.