How can I get a user_reference type field working that allows me to enter names that don't exist on the site as well as ones that do?
Autocomplete username looks like it does the auto complete bit for user registration. It's fairly simple. And could probably be adapted for your UI needs. image annotate looks like it does something similar, but without the ajax.

If you end up making it and publishing it, you can always post it here as an answer.