How about creating a group channel in mix console (call it pre-out) and route all your automated tracks to it. That way you can use the fader to adjust the volume of only the tracks routed to it!
That's one way to do it. Alas, a little unnecessary.
Why not just lower the master fader? You wind up with the same result. (edit - if the chs are peaking, lowering the master fader wont help there)
If you want to do the auto thing, you can do the following... it's a few steps but is pretty simple:
Ctrl+A/Select All > KC for group (or r-click on any event > edit > group)
KC for Automation > Show all used auto
Ctrl+A/Select All again - now all of your events and all of your auto is selected
Ctrl+click on any event; now all of your events are de-selected and all of your auto remains selected
In the info line change the value accordingly
KC for Automation > Hide all auto
Ungroup your events when finished if you would like
The problem there is that this will only work for the channels that already have automation and it sounds like the OP is saying only SOME of his channels already have automation.
I think using 'Trim Automation' might be a better solution...
In the Mix Console, enable Q-Link
Select all the channels you want to change.
Enable Write Automation for all channels.
Open the Automation Panel (in the Project menu).
In the Automation Panel, enable 'To Start' and 'To End' and 'Trim'.
Now, if you move any fader by say -6dB, all of your channels will have their volume reduced by 6dB, including any automation curves that you have already programmed.