Edit several months later: Since I wrote my original comment below, Code Potent’s Update Manager plugin has become the standard solution for our community. It will also be the base for the official ClassicPress plugin directory. There will still be a bit of work required by plugin developers to switch over to the official directory when it’s out, and we’ll make this as easy as we can. In light of this I am invalidating my original comment below.
I understand the idea here, but actually installing the GitHub Updater plugin sounds like a pretty good solution for now.
The reason is that once ClassicPress v2 is out, most (all?) plugins will be hosted on GitHub anyway, so transitioning from ClassicPress v1 + GitHub Updater to ClassicPress v2 should be fairly straightforward. This way, plugin developers shouldn’t need to make any changes or introduce custom code related to updates, and site owners just need to install a plugin once on each site, and then basically all of the “built for ClassicPress” plugins will receive updates without needing to do anything else.