First of all Eric, I don't expect you to add this feature anytime soon. I know you have your own problems to worry about right now, and man do I feel for you.

But while this was fresh in my mind I wanted to get it written down.
We opened up our newest 68C site last night and had a great response. We had offered up a coupon that expired after 50 uses and it was exhausted today. Trouble is, new customers attempting to use that coupon aren't getting told that it's invalid - they just notice the lack of a discount in the final price.
For now I'll just drop a note about that specific coupon into step 3 of the checkout process, but once we get a few coupons out there it's going to get messy. Refreshing step 3 and adding a warning about why the coupon will not be applied to their order would be ideal.