Binding to nodes that do not exist

I have a scenario where I need to display action buttons to trigger a process for 'adding' nodes to an XForms instance (an 'Add New' button, for instance).  However, I'd like for these buttons to be displayed only if the node in question does *not* appear in the instance.  I can't figure how this could be done since the mechanism for binding to instance nodes depends on whether or not the binding XPath evaluates to a nodeset or not.  I could use the XPath 1.0 'not' function (wrapped around the path to the node), but that would only return a boolean value of 'true' which would cause any control/group bound to it to *not* appear (since it's not a nodeset with size).  Is there an easy solution to this problem that I'm overlooking?

Chimezie