It's a complete shift from how you're currently doing things, but what if instead of a boolean field for whether or not they're a subscriber, it's a time/date stamp. This accomplishes a couple of things for you:
- Capturing that they're a subscriber
- Capturing when they opted in
Then, your choice campaigns could be based on in time frame: today instead of filled out form.
If you do this, you'd also need to set up a campaign to NULL their time/date field if they opt out of the mailing.