I dont know your structure database but why dont you add a "unique BBAPI ID" (like a social security number) to each user ?
This way you could allow users to connect to BBAPI with their login + access key OR with "unique BBAPI ID" + access key.
The "unique BBAPI ID" would be visible in your profil where you have to create/change your access key.
This way, no more troubles with non-latin characters !
What do you think ?