Nj,
As the users need the global right "mobile - access", you will know who can use the app.
Little bit of research
In app-system for each login with the app you will see
/niku/odata/login | %24format=json |
you can also see the browser header so you should be able to figure out what types of devices are using the app example (iPad; CPU OS 7_0_4 like Mac OS X)
turns out when ever you see "format=json" in the logs, (inc app-ca.log) it is an interaction with the app
I hope that helps, maybe someone can build a nice little tool to scrape the logs and provide the stats you are after