Resolving custom urls in XHTMLString

I know EpiServer handles internal links in XHTML strin to expand links like ~/link/d9651bd2ab2a44afb74b550b4eb2acfc.aspx to meaningful URLS on runtime http://localhost:57906/en/alloy-meet/

We will be allowing our editors to add links to documents stored in external system, and we will be putting in the url something like extSystem://<document-id> in the markup (raw value in DB) and when rendering, it should be changed to working, friendly URL like: https://externalsystem.com/documents/<document-id>

So I'm looking for a way to identify and parse the links when XHTMLString is rendered. Any place where I could hook? Service to overrride, extend?