Is it possible to make a display mode for surfaces, which shows their current render mesh, mesh wires?
Next best thing would be me just using the custom mesh on objects, then previewing the render mesh which I am adjusting and so on.
I think this kind of display mode could be good for checking topology of objects when I’m doing displacements (as sometimes the same settings will preview different results, depending on the render mesh properties).
Yes you can make your own.
How to from Help:
and a Video:
Mesh wires for mesh objects yes, but mesh wires on the surface objects? I can’t see that option there. So not by adding mesh items to the document.
Basically the rendermesh mesh wires of the NURBS surfaces, underneath. But without necessarily having to actually have the mesh objects added… I don’t know, maybe it’s a crazy thing to request 
Do the commands ShowRenderMesh and HideRenderMesh help any?
Ah perfect! I use extract a lot, but never show/hide.
I’ll just hot key it then. Hey, there’s a toggle too…
Just not connected to a display mode is the only thing. But it’s a shortcut there anyway, so I’ve aliased it.