Nice idea! afaik, the outputs are DC-coupled, so should be feasible. No need to restrict it to the single built-in envelope. Some combination of button presses (that will give Olivier nightmares and fits of the horrors) could be used to enter a mode that specifies the secondary envelope (or LFO) output by repurposing some of the knobs (and the oscillator model indicator LEDs could be used to select what type of envelope or LFO waveform…).
I don’t currently have a Plaits, alas, but I hope to correct that soon.