Nice idea. But AFAIK it cannot work with most "modern" web sites, because they are increasingly using some Javascript nonsense instead of a textarea. (Disclaimer: have not tried this, but some "everything is text" extension years ago)
Thanks for this clarification. (Even pasting becomes increasingly impossible. For e.g. Google does not allow me anymore to paste my username at login, and my (work) username is very long... I suppose pasting might offer an easier way scripted attacks. But yeah, most applications still allow it.)
In general there's always a simple way to bypass these websites limitations, like those who try to disable the right click.
Since you talked about your "(work) username" I thought your employer chose your OS/browser for this particular case.
While my employer does not chose my OS/browser I am in the lucky situation that most of us use Linux with their favorite distro.