Well, one option is to simply not put them in the same hierarchy as your standard nodes.
Aside from that, it would not be very difficult to add a custom "visible=false" attribute to each node that shouldn't be visibile. Then in a SiteMapResolve handler you can walk up the parent nodes from the current nodes and unlink those nodes that shouldn't be visible.
For Menu/TreeView kinds of controls, you would need to add custom binding code that removes those nodes. It's trickier in this situation because you have to make sure to add the child nodes to the parent's child nodes collection.
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Danny
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