There's a version of this using a directory of images and loads in a bigger picture if you zoom in: http://fhtr.org/multires/ (Note that, yes, it'd be better to have a tile map for large resolutions and load in just the visible part of the image. And dump the hi-res tiles when zoomed out.)
SPIF's intention was to throw out a "it'd be cool if browsers supported something like this natively"-proposal, as the browser knows best what pixels of an image are needed for sharp rendering. For the webdev, the experience would be to just put the image on a page, rest assured that it looks good. Like with SVG.
Yes, loading JPEG2000 / progressive JPEG with stream truncation would be nice.
Images don't load on iOS? Probably some silly bug in my code.
Images can't be saved with right-click? That's probably due to using revokeObjectURL after loading the image from a blob.