After thinking about your suggestion and Lou's comments, and after some more exploration of the presentation APIs, I think that the most basic functionality (connecting a TEI viewer to a IIIF image service) could be done via smart pattern matching as you suggest.
For a more complete solution (connecting to the IIIF Presentation API endpoints for a page or zone), I think we'd need to extend the TEI to allow an encoding to specify the following:
* The Image Service URI
* The Image Server compliance level
* The IIIF Canvas URI representing that page
* The canvas context (i.e. IIIF Manifest URI) the page is located within, since IIIF Canvas URIs need not be dereferenceable (don't get me started...).
All of these would require logic on the part of TEI presentation software to use them effectively, but should also be backwards compatible with existing systems if the value of facs is a URI pointing to a maximum-resolution image.
Am I missing anything?