Update, if anyone cares: yes, routing the AD Out through an attenuator and patching it back into the Attack or Decay input allows to gradually make the Attack and Decay time a huge lot longer. The downside is, the longer they get the less smooth they work, and get closer to being on/off rather than fading in and out just as nice, at least in regards of their tail.
It also works in the opposite direction, you can make the AD Envelope shorter than it's initially possible by just patching it back into itself through the inverter.
Using other 5V CV sources would work just as good I guess, I don't think there is a need to use more voltage than that, the possible internal results already are quite extreme by normal standards.