Hi Mike,
I've been using the API, and I believe it's only currently possible to
efficiently query a user if you know the user's ID.
You 'could' iterate through every user ID, but this isn't efficient,
and you'll need to implement a queue so that you don't hit rate
limiting issues. This isn't a viable option in the long term, as the
user base will increase over time, lengthening the time of your
search.
I'm guessing this is a call that will come to the public API sometime
in the future (but I'd look to the development team to advise if and
when this might be).
Regards,
Jonathan
@mobijack