Hmm, nope! It seems I was fooling myself because of the Material I was using for my character mesh, which is translucent. When I applied an opaque Material, the character mesh behaved quite the same as the rest of the geometry: the 3D Widget overlaps it, like the custom depth buffer wasn’t even used.
Back to the drawing board! Will reply again if I figure out a solution.
Incidentally, here’s what I have currently: 3D widgets showing how they both occlude an object in the background (that’s to be expected), as well as an object in the foreground (behaving like a generic UI element). You see how my ‘character’ model (i.e. a large hand) passes through the UI element, but not in front of it. Instead, the custom depth buffer ‘reveals’ part of the ‘UI Colour’ node you show in your post, exactly where the Mesh intersects the UI element. I feel like I’m very close, because this would seem to be the exact opposite behaviour from what I want. Now all I have to do is figure out how to… flip it.
