Hi Dan,
yes, you should be able to develop a Mobile add-one for a Moodle local plugin.
I placed the documentation of the new db/mobile.php there file because It was the only page where I saw the typical Moodle plugin required files listed (please, let me know if there's a better place).
You can add any Mobile add-one to any place where is a delegate (side menu, course menu, user profile...
Please refer to: https://docs.moodle.org/dev/Moodle_Mobile_Remote_add-ons (for instructions for developing the remote add-on)
And
https://docs.moodle.org/dev/Moodle_Mobile that is the generic Mobile docs
We'll really appreciate the feedback from an experienced developer like you, so please, feel free to post here any suggestion, feedback, rant or whatever
Juan