I've found the solution to an old problem: how to get the request URI into the output. I can pass it as a parameter into an XSLT transformation like this:

<map:transform type="saxon" src="xsl/highlight_matches.xsl">
  <map:parameter name="browserURI" value="{request:requestURI}?{request:queryString}" />
</map:transform>

It doesn't include the protocol, server, or Tomcat port, but those are not important anyway, really.

I'm also beginning work on the code which highlights the phrase matches through XSLT. Actually, it seems most likely that this will be achieved through a call to a Java class, as documented here.