ModEchoVerb. It's capable of many types of effects. But it also offers two choices for infinite hold [DECAY, and ECHO_FDBK], that can be mixed together. This preset starts at an INF default, and can be HotSwitch'ed to 5.00 DECAY time. It's a reversed approach: one that begins with an infinite drone, and fades out when switched 'ON'. The expression pedal takes the single 1.5 second ECHO, and sweeps it to infinite ECHO-FDBK . At the same time, FLANGR MIX sweeps from 0-100 to introduce deep modulation. Blending HotSwitch & expression pedal positions will combine or separate these two independent 'drone' types.
A very workable stand in for the Electro Harmonix Freeze Pedal. The flex switch or whatever you want to map to it controls the audio you grab (requires an external switch). I'd gather if you can map a midi switch to it you could even get the momentary type function going.