Ohhh, sale conditions. That's what I was searching and missed.
Depending on the time scale and when projects get set to pwyw, the interface would just not differenciate between a minimum price customer or a pwyw customer. So a coupon code as a individually priced text file might do. Honestly, who is gonna support indie games and would cheat on discounts? One can limit the number of sales through this coupon to 2x the number of customers or some safe number. And hey, if the thing is so popular that people try to cheat a discount just to read it a few weeks earlier, at least it is still money they pay, instead of waiting and reading it for free.
Sale conditions
It’s possible to restrict who has access to the discount provided by your sale using a Sale condition. They currently come in two forms: coupon codes and ownership requirements.
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From the sale edit page, select Add condition and choose the project that must be owned for the sale to be active.