Well, RSS is an open standard, and the URL for this annotations feed is public knowledge, so anyone who wants to aggregate these annotations can do so. The people doing Mono have everything they need to implement something exactly similar. There's nothing closed about this annotations system.
Of course most of Mono is mostly just a clone of .NET, so it's not really clear what benefit there would be in offering their own documentation web site. And as far as I can tell, they don't. It's obviously a lot less effort for them just to rely on Microsoft to provide complete documentation for the .NET Framework since most of Mono is a reimplementation of that.
So look at it this way: if you were to ask the question:
"Where do I go for documentation on the classes and services provided by Mono?"
the official answer is apparently: Microsoft's site. So if you're asking where the annotations for Mono are, well given that all the documentation for the class libraries in Mono is already on Microsoft's site, there's your answer...
What might be interesting would be for someone to provide a Mono-specific feed for annotating Microsoft's site. Feel free to do this - it's an open architecture and anyone can provide whatever annotations feed they like. I haven't done this because I'm more interested in the new functionality that's coming in future versions of .NET than I am in re-implementations of functionality that has already been around for a few years - Mono is not technically very interesting to me. It might be an interesting technical, political and social phenomenon, but technically speaking, it's just a rehash of a bunch of stuff that has been done before.
And as for the things that aren't just a copy of what's in .NET (the Mono-specific GUI stuff for example) then I guess they could build annotation support in exactly the same way.