If you want mages to register with you I suggest you offer incentives rather than try to force them too when it isn't even a legal requirement.
You're quite a skilled mage so you could perhaps offer to assist them in their research and such.
Such as I help those in my guild with their projects as they assist mine.
I'll still need to know what you need for the Warforged shell. It's also possible I'll need to be named by the Church as the High Priest of Northern Cormyr to use Beatrice.
You can aid me in such by helping the Church gain prestige and influence in the city.