One of the problems here is that the library you mentioned is commercial, so it cannot be used for official Sparkle. For dynamic content you can already do it, just checking the Accept-Encoding header and processing like you do. Or, copy/paste the compress middleware source code and create your own that supports Brotli.
Of course, it would be ideal that Sparkle provides this built-in, but as I mentioned, we would have to implement the Brotli algorithm ourselves, which is a kind of reinventing the wheel. But something to be considered, sure. Maybe add it as a feature request so we can track for the future?