While I think this a a worthy suggestion, I also believe that "hidden" gestures that have low "discoverability" should be dealt with extreme caution.

For example, in a reading app, a pinch gesture can also be assigned to changing font sizes, and perhaps that's what many users will expect to happen.

I think keeping gestures to a minimum (in this case, just a tap, which coincidentally, is a feature of many e-book readers) serves Instapaper well.

By the way, well done Marco. Solid update, worth working hard for, and I am sure your customers appreciate it.