XSLT, using a string value as a node-set() param

Hi everyone,
Apologies if this is off-topic..

I am writing an XSLT where I want to pass an XPath statement in as a parameter.* Like so:



Note the single quotes around the XPath - the param is a string literal, not a node set.

I then want to go:




But ASP.Net doesn't like this at all, in fact my web server crashes as soon as it attempts the transform. XMLSpy doesn't like it either. It works fine when I declare the param like this:



Can anyone tell me if there's a way to get it to do what I want, or a workaround?

As background, I'm doing this because I need to calculate the XPath on the fly based on the current URL. It's basically a kind of URL rewriter - I'll convert /mypages/mypage.htm to /pagegroup[@title='mypages']/page[@title='mypage']