Hey Axel,
Not sure on the on the over-writing of the output path, it's probably possible by over-riding one of the standard meta data variables but no official support for it at this time.
However, from what you've said on IRC you mainly just want the ability to specify a custom URL (or many custom URLs) for a document. You can do this easily by setting the `url` (string) and `urls` (array of strings) meta data variables.
E.g.
---
title: "Some document with a custom url"
url: '/my-custom-url'
urls: ['/also-support-this-one','/and-this-one']
---
Does that help? :)