As followup: QChris did the actual work of reading the relevant RfC for me, and for now I've settled on:
WikipediaApp/<version> (<OS>/<version>; <form-factor>)
https://gerrit.wikimedia.org/r/#/c/123117/ is a WIP patch for adding this to Android, I suppose an iOS patch will come later. Checkuser work will probably take a bit more time to materialize.
Thanks everyone for chiming in!