Mediation Router has no built-in component that can do this for you, but you can combine some building blocks to achieve this goal.
If you write your own Processor, you can do whatever you want in the process method. If you reuse the existing XsltBuilder class iside the proces(Exchange) method, all you have to is set the transform source (your XSLT attachment as a Source) and call the process() method on the XsltBuilder.
In your RouteBuilder, you just add a process step, refererring to your own Processor.
I took alternative approach and started to use servicemix-saxon component which I call using JBI.