Can’t you do this by using an expression pedal to set the decay length of a reverb to infinity? To quote Brock from a different thread:

“Most of the algorithms featuring reverb have INF and FRZ positions in the DECAY parameter.

 Infinite continues to layer your input through the reverb. Freeze takes whatever is currently in the buffer, and holds it (no new input). Latch or unlatch the INF and FRZ features with an Aux switch, HotSwitch, expression pedal, or over MIDI.”