Expressions based on page count

I'm currently working on a stylesheet for a manual.

In this case I must see to it so that the inside of the cover pages dont contain any information at all (the cover will be printed 4+0).

This is easy for page 2 since you can define "Start new: Odd page" for the following page set which then will begin on page 3 and leave page 2 empty. But i dont know how to do this with the second to last page. My thought was to create a xpath-expression that based on if the page count was odd or even would add one or two empty pages before the last page.

Is there a way to calculate and use page count in xpath-exressions?

Is there some other way to solve this?