It seems like that would be possible. The only restrictions that I can think of are the fact that each (iOS) extension needs its own app with significant functionality (so this extension would have to be provided by a file browser or something), and a extension will only run for as long as it takes to do whatever its supposed to do (so no background extensions). I think as long as you're watching the video the extension will be active, so that shouldn't be a problem. You might need to add support to the video player, but besides that I don't see why this wouldn't work

As for the GPU thing, I have no idea, but it seems like they're trying to be more open; I'd be kind of surprised if they said no.