The "Get ready for iCloud Private Relay" session[1] from this year's WWDC makes it seem like there will be a publicly available list of IP addresses used by Private Relay:

Private Relay guarantees that users can't use the system to pretend to be from a different region, so you can continue to enforce region-based access restrictions. Details about the proxy IP addresses will be available as an article associated with this session.

Though I haven't been able to find the aforementioned article.

[1]: https://developer.apple.com/videos/play/wwdc2021/10096/