switch toggles conditional on XPath expressions?

I'm a complete XForms novice, so please forgive me if I am missing
something considered "obvious".
 
I would like to toggle a switch based on the evaluation of an XPath
boolean expression, but I don't see a construct of XForms mechanisms
that can accomplishing such.
 
The effect I'm trying to achieve is this:  I want form controls to only
be visible when an arbitrary XPath boolean expression evaluates to true,
analogous to the "readonly" property that may be applied to model items,
however I want to ensure that an associated form control does not appear
on screen at all, not simply rendered disabled.  These controls would
appear / disappear as items in the data model take on new values.
 
My original thought was to surround each control with a <switch> element
and have an action respond to all value changes which would
conditionally (based on an XPath expression) toggle each control to
"appear" or "disappear".  Unfortunately, I'm unsure of a way to achieve
the conditional toggle.
 
Is there another way to accomplish such an effect?
 
Any suggestions would be most welcome.
 
Thanks,
 
  .: jeremy.johnson@imtc.gatech.edu :.
.: interactive.media.technology.center :.
  .: georgia.institute.of.technology :.