Set up a Polywork Custom Domain
Fiddled with DNS
Set up https://polywork.0az.io!

As it turns out, you don't need to set up Cloudflare Workers, or fiddle around with the Host header, like the custom domain docs imply. All you need to do is disable proxying in the Cloudflare dashboard for your alias.

(I don't have anything else set up for that domain… yet!)