GeoNode Shape Keys let you deform linked and overridden meshes using sculpt mode, for example to make small corrections to an animated character.
The add-on's UI is only visible on linked and overridden meshes.
It can be found under Properties->Object Data->Shape Keys->GeoNode Shape Keys:

You can then hit the Add button.
Once you press OK, you can start sculpting! You're not actually sculpting on your original mesh. A duplicate has been created for you, and the original was hidden. You can switch back to the original using this button:

Removing a GeoNode ShapeKey entry will also remove the relevant modifier and object. Don't remove those in any other way.
If your mesh doesn't have any overlapping in its UVs, you can just use that. Otherwise however, you should create a separate UVMap for use with this add-on. The UVMap is used by the GeoNodes set-up to match the vertices of the real object with the local copy that you sculpt on.
The add-on also allows selecting multiple objects before pressing the + button. In this case, the sculpt object will have all your previously selected objects. The principle that the sculpt object shouldn't have any overlapping UVs still stands. An operator is provided to easily create UVMaps across all of your asset's objects without any overlaps, for use with this add-on. You can find this here:

This extension requests the following permission:
To load the necessary GeoNodes from its .blend file
Absolutely loving this addon and have been using it a lot for personal projects and shortfilms. It's a fantastic way to workaround a big limitation of Blender's Linking system.
But I honestly think it's super powerful outside of that as well. It allows you to create shapekeys "after" modifiers relying on Rest Position like Corrective Smooth and more. Which is why I'm wondering if this addon could be enabled for non linked and overridden objects as well. Since I personally see a lot of usage regarding this addon outside the overriding workflow too.
Heya, glad to hear you found use for it! Yeah, letting it work on non-linked objects should be just a matter of not restricting the UI, I wrote a note down in case I ever update this add-on.
Great addon, but which key do I press to toggle the view of the original mesh? It's blank in the "about" section for some reason. I need to do fine-tuning on an animated pose, and it's nearly impossible working from the T-pose mesh.
Hey, this add-on doesn't come with any hotkeys currently. There is a button in the Properties Editor->Object Data->Shape Keys panel, which lets you switch from a shape key storage object back to the original object. You could probably right click that and choose Assign Shortcut.
Its exactly what I've been wanting to have in blender for a long time!, it has a few quirks that would be great to iron out, tho.
In my rig, there are multiple objects, one of them shift a couple of units when I create the shapekey so its hard to do the sculpt accurately. In the other object took me a while to get it to work as it would work great while sculpting but then when going back to the animated object it would give me a scrunched up mesh. I see it depends a lot on having good UV maps. Any chance you can make it not depend on UV map? Other than that kudos ! <3