This is tricky, but one idea would be to use Recordings, see...
https://github.com/37signals/highrise-api/blob/master/sections/recordings.md
...so you could work your way back through recent activity using this, just pulling out emails and ignoring the notes and comments.
I use the same wrapper you mention, and it's probably the best route really, if PHP is your lingua franca. That said, I don't think it supports the Recordings API functionality, though it would be quite easy to add it to the feature-set.