I don't know if this is the proper way to go, but my feeling is, better a few comprehensive plugins than many specialized ones, so I've added tag checking to your plugin... forked on github, the source is here:
All it does is check your reputation (unless you are expert level), and if you don't qualify, it gives you a "button", rather than a submit. When you click the button, if one of your tags doesn't exist, it returns an error. If they all match, it calls .submit() on the form.
As I think about it, you could still submit the form by editing the page in, say, firebug. To really stop new tags from being submitted, you'd have to edit the core, I guess. I doubt anyone's really that paranoid about new tag submission, though, and this works for most cases, I think.