But apparently you can make use of the ScriptSrc attribute to reference files from the Virtual File System (e.g. a Document Library in a site) as well! (thank you Frederic Dobon for mentioning this!). The trick is to use the ~SiteCollection prefix in the ScriptSrc attribute, for example:
It’s that easy! For your convenience I’ve uploaded the source code and the resulting SharePoint Solution (WSP file) to CodePlex.