Sounds good to me!
Naturally, I wish that it could simply re-render with the rest of the page/window without interruption, but if it's in the browser vendor's hands, then I suppose it's out of yours.
Is this a limitation across the board, or is it limited to specific browsers?
As far as IE goes, is this something that you could ask them to put on their to do list (and maybe even work with Khronos on getting this fixed across the board)?
As I said at the top, it actually is useful on some of the underpowered laptop GPUs that I have at my disposal to load at a low res and then fullscreen it after that in order to maintain a higher framerate.
I can't quite remember as I write this, but does OpenSeadragon always render at current res of the viewer, independent of what WebGL is doing?
Side note: In the large gaps between photos in poor reconstructions the huge pixels sometimes seen appear to be camera frusta rendered at super low res close to the viewport.