This is not a feature that I wish for because rooms and players are different. One Volume setting for all, would not work in general.
One approach would be to allow a rapid double touch of a Volume slider to propagate this setting to other members of the Group.
correct, I would not want it to lock in the other devices "This would be a quick adjust setting (not a permanent toggle) so existing functionality to adjust per device or as an entire group is retained."
What you mention as a rapid double touch would be perfect.
If you quickly set the ‘group’ slider control to the zero volume position and let go for a moment and then slide it back to the desired volume position, all the rooms in the group will be set to equal volume. That’s almost as easy as a double touch on the group slider, perhaps
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Well now I feel dumb for not thinking of that before. Solved, thanks @Ken_Griffiths 
If you quickly set the ‘group’ slider control to the zero volume position and let go for a moment and then slide it back to the desired volume position, all the rooms in the group will be set to equal volume. That’s almost as easy as a double touch on the group slider, perhaps?
Simplest solution is always the best!.
The simpler solution works if all units put out the same sound levels at the same positions of the volume slider. Different Sonos models often do not do this and the solution to that is to first get this normalisation done by setting different max volume limits for each unit. For instance in my case, play 1 units need to be at 80% max limit to equalise sound levels with my Connect Amp driven zones - the latter deliver the same sound levels as play 1 units at higher volume slider settings.
@Kumar you bring up a good point. In a perfect world, that volume normalization between different rooms could optionally be handled by TruePlay at the preferred listening spot for each system and a set volume, i.e. TruePlay adjusts the volume slider so that all systems are at the same spot for 100 dBa. I’m sure there would be descrepencies between systems and how linearly they increase volume, but it’s a start. Or maybe TruePlay already does this? I only have Android so I can’t use it.
The “discrepancies” arise because different models put out different sound levels at max on the volume slider, by design. And the sound levels from Amp are all over the place because these are determined largely by the design of the attached speaker downstream of it. Of which there is a very large variety.
I suppose Trueplay could do this, but not as a part of the existing tuning per speaker, because some may not want both outcomes. Today, Trueplay only does what it think it needs to in order to make the target speaker or speaker pair to deliver best sound quality, regardless of volume levels.
But the max volume limiter feature does the same job quite well, perhaps via a few iterations that are a one time thing. How loud the speaker sounds is also decided by the nature of the room and the listening distance, so it is better left to the user to do this normalisation via the provided feature in his/her listening space; once done, the recommended solution for getting groups to play at the same sound levels is very effective, I have found.
The “discrepancies” arise because different models put out different sound levels at max on the volume slider, by design. And the sound levels from Amp are all over the place because these are determined largely by the design of the attached speaker downstream of it. Of which there is a very large variety.
Trueplay isn’t allowed with an amp, unless you’re using Sonance speakers designed for Sonos. As you stated, Sonos doesn’t know the characteristics of the speaker. The same goes for the Port.
