Photoshop :: Shadows/highlights As A Smart Object

I have recently read about and successfully tried applying the Shadows and highlights adjustment to a layer that has been converted to a smart object. This is to retain the shadows and highlights adjustment for further tweaking at a later date, rather like an adjustment layer.

I found however is that one is prevented from working on that layer with say in my case the stamp tool (and other but not all tools), unless the layer is rasterised. This of course causes the shadows/highlight adjustment to be incorporated in the layer and lost to further adjustment.
Is there no way to apply shadows and highlights permanently in a non destructive way, which can be tweaked at a later date?
I am using CS3 Beta on Windows XP Home.