It's pure chance I happened to log in here and stumble on this thread. Haven't been coding for months.
I do have a Code-Mirror editor for templates and plugins (and other admin textareas) for vB4 but it's part of a file manager project I've been working on. I could copy out the required parts I guess and make a product out of it. It will involve a direct file edit in includes/adminfunctions.php.
I don't know anything about vB5 so you're on your own there.
This could take a day or so. I have two different versions and I have to check out which is best. And i'm veeeeeeeeeeeeeery rusty at coding.