One-off payments are always on our radar. In general, we love the idea. At the same time, we need to ensure that we stay true to what Patreon is all about, which is building a membership platform for creators.
For now, the best way for a patron to support a creator with a one-time pledge is to become a patron, and simply delete your pledge after your been successfully charged or after the first of the month. You'll know when that happens because you'll receive an email confirmation right after the 1st of the month or when you've been charged that says your pledge has successfully processed.