It's easy enough to make the coupons exchangeable for more promo items (such as stickers, tshirts, hoodies, etc), so they're never worthless, exactly. You just might not be able to buy the project item, and only get really overpriced shirts instead.

(You could even make mod versions with "failed" on them after the fact, and use the coupons to buy those.)

Naturally, you should have giant, red, all caps letters explaining you're funding development, and might never accomplish the project successfully. Coupons only cover items posted for sale in the store, etc etc.

I'm mostly just curious about it as a legal method to give the reward at a tier without promising the exact reward.

(Side note: is this also an end-run around the multipack rule?)