Global sculpt layers is very interesting. I didn't even consider that it could apply to multiple sculpt objects. To be honest I can't say which way would be better. Global provides a lot more flexibility, but harder to keep track of what's what.
Maybe if sculpt layers are global, then we could achieve something more localized with the addition of 3b files embedded into the voxtree that we can 'dive' into and while inside it, all sculpt layer changes will be local to that 3b file. Please forgive the next portion, it's just some ideas that I've had before that are starting to come into clearer focus with the addition of sculpt layers.
So here is what I was thinking...
I think it would be great to have the photoshop smart object layers equivalent in the voxtree... .3b files embedded into the voxtree that live alongside the voxel and sculpt objects. They wouldn't be directly editable like vox/surface objects. Since they are 3b's they could show objects in their sculpt state (vox/surface mode), retopo state or paint object state. And they could 'dived' into like a sub-object in illustrator and indicated via UI something like this -- main scene > 3b object 1 in order to keep track of what level you are at in the project.
Further, in addition to Smart Object Layers, I would also propose a Quad mode.
So essentially, 4 Voxtree layer styles in the sculpt room and render room.
Voxel mode - for freeform sculpting/booleans
Surface mode - for liveclay/remove stretching/booleans
Quad mode (with subdivision stepping and supported sculpting tools) - compatible with the models tab and works with merge tool and curves for placement - even if this mode isn't automatically generated such as surface mode/voxel mode - this mode could support more basic sculpting tools...draw, pinch, inflate, smooth, etc. along with some of the selection and retopo tools - Would also provide a non-destructive means of increasing/decreasing resolution without losing details...would ptex work here? If so, sculpts could be painted in paint room with SculptLayers and Smart Materials then baked later to UVd mesh. Pin retopo mesh to sculpt is an interesting workaround, but it isn't very efficient (rendering/editing both retopo mesh and sculpt mesh at the same time) and not all the sculpt tools are supported.
Smart Object (not editable outside of transforms) - represents 3b objects - can be non destructively displayed as sculpt mesh/retopo mesh/paint object - compatible with the models tab and works with merge tool and curves for placement - can be destructively converted to quad mode/surface mode/voxel mode for editing (this would only be a destructive step in the the scene you are in. The instanced .3b file saved to disk would be unaffected)
Doing this would open the way for powerful workflows for concept art which 3D Coat is already being used for a lot and also make 3D Coat more production friendly by allowing layout artists to arrange scenes and work on assets in place or referencing in 3b files that other people are working on asynchronously. Ultimately, the whole scene could be exported non destructively to another application...be it a game engine or 3D animation tool with everything worked out and in place with no duplication of work required.
My 2cents...apologies for changing the discussion...just putting this out there for roadmap purposes in case any of these things that you've been thinking of addressing.