> There seems to be some idea, that it is possible to use firebird
> stored procedures to populate user interfaces and then take the
> modifications and use them to update a database.

Yes, this idea is not new.

> This method seems unreasonable and impossible to me.
>
> Has anyone else done things this way?

I'm always design client-server applications with rich GUI in this way.
Users even have no grants to SELECT from any table. Applications calls
only SPs.

Regards,
Vlad