Okay, how's this for esoteric?
Deep within Drupal's filtering system is a system variable (in the Drupal sense) called "filter_allowed_protocols". This variable is an array of system protocol prefixes ("http", "mailto", "ftp", etc) which are to be considered "safe" for filtering purposes. However, there does not seem to be in core a way of editing this list to add new protocols which you think should be allowed and not filtered - "internal", for instance. So I propose a simple module which would allow you to add new items to this list (or remove them). My proposed name: Allowed Protocol Editor, which gives you a rather fun acronym. Ook ook.
But maybe I haven't searched long enough and a module which does this already exists. Anyone know of one?