I'm thinking Xaoc Devices Zadar and an Intellijel 1U Quadratt to attenuate or attenuvert the outputs manually.
I believe the Zadar can loop and play at very slow rates. The Quadratt ensures you can adjust the amount of modulation on the fly. You could also use the Quadratt to mix waveforms from the Zadar as well.
I think that would be fast to set-up in a pedal board situation.