I wonder what the overhead of doing this as Rack Middleware is versus just putting it into your layout.
This is markup, it belongs in your layout files. You're really just disorganizing your app by putting this into middleware.
I think the biggest disadvantage of putting something like this in middleware, is that it's completely incompatible with stuff like memcache. The injection of this meta-tag is done after a memcache SET would have been done in the web app, and so if you have a reverse proxy in front of your webserver, for any cached page that's served, it won't include the injection. Also, the user-agent check that's being done in this plugin yields incorrect results in the presence of caching for similar reasons.