Having given this more thought, I think it can get really complicated for non-English Q2A sites, since the set of acceptable characters will be different. So...
I expect Q2A 1.5 will support a new type of plugin module called a 'filter', which can modify and/or validate user input of various types, before it goes into the database. Assuming this happens, I'll make usernames one of the types of content which can be modified/restricted in this way.
Hopefully it's a short distance from that to a plugin which provides the restrictions you're looking for.