Sorry, but I don't think you're getting the point here. An 800mm lens design is going to have an image circle larger than a FF sensor and thus larger than an APS-C sensor. A 500mm lens is still going to have an image circle larger than a FF sensor and thus larger than an APS-C sensor. You cannot 'design a smaller, brighter image circle' for a supertelephoto lens. The image circle is not limiting for FF, so it's not going to be limiting for APS-C.
Ok, I get that what you're suggesting is that Canon effectively design an 800mm lens with a speedbooster built into it. But that design would yield 500mm f/7.1 and would still cover a FF image circle, so what would be the point? A simple 500/7.1 would be a lot cheaper to build than a speedboosted 800/11, and it would be lighter, too (less glass).
There are no long telephoto lenses for APS-C. None. Even if the above point about the image circle not being limiting is not clear to you, you should think about why no one has made one – if it made any sort of sense, an OEM or 3rd party lens maker would have done it. Even with brands that only make crop sensors, like Oly/OM, their lenses that go beyond 300mm would actually work on a FF sensor. Because the image circle is not limiting.
The issue is that there is simply no point in making the lenses you are talking about. Not just from a cost perspective, but from an optical physics perspective.
This is also true, of course. It's likely why there are few high-end APS-C lenses from Canon and none for the RF mount, and why there has never been an L-series APS-C lens (though they do put them on fixed-lens camera with smaller sensors, like the PowerShot Pro1 and several camcorders). But that is irrelevant in this case.