The thought had crossed my mind, but a user might delete macros hosted on GitHub/Dropbox, etc, which isn't great for future referencers.
I suppose, in an ideal world, we'd have a shared KM repository, with versioning. Perhaps that's possible with GitHub, but Peter would have to design an interface.
Then there's still the issue of how to check for those updates. This method works, but it's a bit fiddly.
@griffman has done some fantastic work on a forum bookmarking tool. I wonder if he has any thoughts about this...?