I'm down to contributing to such a project. But there wouldn't even be a need for scraping, just crowdsourcing: people can submit coupons on checkout (perhaps try to detect if they are using one, at least on popular sites) and be offered to share them.

Once shared, they are presented to other users, who can try and validate them, or indicate they don't work (for single-use coupons or expired promotions). "Bad" coupons get removed, good coupons keep getting presented to users.