Cannot load xsl from _layouts in the 12 hive using DataFormWebPart

I'm trying to store intranet wide XSL file in the 12 hive so that it can be accessed in any site collection without duplicating that file.

I'm using a standard DataFormWebPart which I switch from using the XSL file within the local site (which works fine) to the _layouts relative URL which is when the usual error occurs:

Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Windows SharePoint Services-compatible HTML editor such as Microsoft Office SharePoint Designer. If the problem persists, contact your Web server administrator.

Is this by design or am I missing something blindingly obvious?