I'd like to chime in and say this has been an excellent thread. Having a builtin magnifier is a terrific addition to any website using MSS, older printed matter, and individual illustrations where detail is necessary or merely nice. The magnifier does, however, depend on available high-definition images, so perhaps a two-step interface may be necessary, as with the set of 1,230 TIFF page images I want to make available with a transcription of Hall's Chronicle (1550): thumbnails clickable to full page images, with magnifier. Sounds idea. Now I just have to figure out if I can do it before the end of time.
Cheers, Al Magary SF