I have a webform that contains a date field. I need to run some code that checks to make sure the date is not already in use (i.e. that no one else has submitted a webform with that particular date). I already have the webform validation module installed, and I know I could use that (create a custom module with custom validators), but the validations don't run until the form is submitted. Because there's a required file upload, I'd rather be able to allow the user to check if the date is available before doing the upload.
Is this possible? How would I go about it?