With PNG24 you'd at least get better colour depth. In which case why not use the same Dispose None technique in APNG (https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_...) and a polyfill (https://github.com/davidmz/apng-canvas) to provide cross-browser support?
Actually it might be preferable to always use the polyfill if you want to do something like the Apple thing where you can scrub around the video. Still doesn't have I-Frames to do frame-skip during fast movements though.
The smarter encoders would encode most of that rectangle as transparent and let the previous frames show through.
You can update multiple rectangles each with 0ms delay. Issue is that most browsers don't like dealing with 0ms updates, and will consider rectangle update a frame update. Some browsers will silently increase delays. 
You can pause once all necessary rectangles for frame have been updated. 
You can have full colour image provided that it is built from multiple rectangle updates, where each rectangle contains its own colour map. 
 : Browsers not respecting GIF rectangle update delays : http://nullsleep.tumblr.com/post/16524517190/animated-gif-mi...
 : gifsicle [image] --delay [delaytime] #[start]-[end] : http://linux.die.net/man/1/gifsicle
 : True Colour GIF : http://phil.ipal.org/tc.html