You could try something like this in the Gate fields of the ADSRs:
1 randExpGate: !AverageDur s minDur: !MinDur s maxDur: !MaxDur dutyCycle: !DutyCycle
where some of the EventValues could be replaced with constants if you know in advance how quickly or slowly you'd like things to evolve and whether you'd like silence between the gates, for example
1 randExpGate: 5 s minDur: 3 s maxDur: 15 s dutyCycle: 0.75
Is this along the lines of the results you are looking for?