I used to have an old Gibson amp that had a tremelo effect with intensity built in to it.
It would pretty much do exactly what you are describing (other than you couldn't control the rate with an expression pedal, though putting the pot into a pedal would have accomplished that.).
I always wanted this effect, combined with an Attack delay, followed by a delay so that I could simulate EVH's "Cathedral" and Night Ranger's "Rumors in the Air" without all the volume knob work on my guitar.
Just get the right tempo and pick the note during the kill phase, and let the attack delay do the volume swell for you while the delay does the repeats for you.
Found this thread after using the Tremolo effect (10 depth, minimum crossover, 0% stereo, set up as a momentary morph) and that's as close as I can get for now.
I did put in a feature request for a "shape" parameter, since it defaults to a sine-wave curve. They could add a square shape so we can get something closer to the killswitch effect. It'll work for now though -- or I'll just use my actual pickup switch.