Nope. Neither will work. Sorry. That's what I get for speaking b4 thinking. Where's the variable going to get it's value from? Certainly not from the <fo:page-number> instruction -- that's for the FOP processor to use not the XSLT processor. You'll get NaN.
No. The value is an RTF with a fo:page-number element as its only node below the root.
J.Pietschmann