That feels like the wrong place to do it. The simplest thing would be to just pop up a JavaScript dialog that intercepts the form POST, and ask then.
This does exactly what you want with the exception that it'll ask them whether or not the form is valid, but I think that's irrelevant.
If you do it in JavaScript before submitting the form then your Python code stays cleaner. Plus, if you did rig up some hideous hack that worked as described in the original question, you'd suffer any time you tried to write code that imported or used this form -- including unit tests.