Hi Phil,
Thanks for reporting and describing your issue.
While described issue is closely related to the other one from development stand point it makes more sense to address the source of the problem rather them to spend time on workarounds of the current solution.
At this point, we have worked out the solid plan for approaching most of Username related issues (if not all) in this
Validation for User Login field discussion and have a task ready. Mostly likely it will go into 5.1.3 release (my suggestion), but patch might be ready before that.
In case if it's super critical to your project, we can give you a temporarily solution when Users won't be able to register with NON alpha-numeric usernames, but it won't be a complete patch for that task since there is more to it.
DA