I think some of it depends on which finger you're targeting, too. Wearing it on the index finger makes it easier to tap/push buttons using the thumb.
In my HCI class, we went over this paper [1] (2010). It has a wide variety of always-available input and output and many of them would work well with these glasses. My hardware crush is a Myo [2], although I don't know how well it works. The videos of it look neat, at least :).
Also, I had the exact same idea for a hand interface as a project for my Ubicomp class, but I didn't have the skills to do it. I ended up doing research for "smart jewelry" instead :|
[0] https://www.kickstarter.com/projects/1761670738/ring-shortcu...
[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...