As you well know you can only select one envelope. I tried making a script long time ago but ran into some weirdness so ended up with simple/dumb custom toggle action, downside is you need to call it twice if not all envelopes are/aren't in lane, good enough for my use,
Code:
Custom: Toggle all envelopes in lanes all tracks
SWS: Save current track selection
Track: Select all tracks
Envelope: Toggle display all visible envelopes in lanes for tracks
SWS: Restore saved track selection