I was thinking along the same lines, like it could be implemented to make one-time purchases to unlock in-game items, areas, or encounters, based on a transactional prompt. But, reading more about the spec (which is interesting, and makes sense for a lot of web content, like blogs or articles) it seems like the user can either be actively streaming payments to you, or not streaming them. There's no way to demand a specific one-time payment.
The response on this thread indicates that Coil only streams a specific amount per second that cannot be configured. I can't confirm yet because I ran into a javascript error when trying to sign up for Coil account, and have emailed them about that. I don't think there are any other providers right now, but I wonder if that's a limitation of the web monetization spec, or just how Coil has decided to implement it.
So anyway, if you wanted to let the player unlock things when a certain amount has been paid, it sounds like the best you could do is keep the payments streaming, and keep track of the total amount paid over time, and then use that to determine what they can access.
Another thing you might do, since you can start & stop the money stream programmatically, is have areas, challenges, or other content that a player must stream payments for while that content is being accessed. Maybe you could have bonus areas that allow a player to "catch up" on some in-game resource in exchange for streaming micropayments. You could even put a time limit on those areas to maintain game balance, and stop payments when the time runs out or the player leaves.