I'm gonna go searching for it but, in the meantime, anyone know the process for submitting a hostname to be added to any of the lists used by PiHole, et al.?

The hostname that Ubiquiti is using -- trace.svc.ui.com -- seems like exactly the thing that should be blocked, IMO.

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FWIW, if you're using PiHole and want to block these access points from "phoning home", you can simply do the following:

  $ echo server=/trace.svc.ui.com/ | sudo tee /etc/dnsmasq.d/ubiquiti_access_point_phone_home.conf
  $ sudo systemctl restart pihole-FTL.service

This will cause dnsmasq, the underlying resolver, to return NXDOMAIN for any such queries.

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EDIT:

Apparently the "pihole" utility has functionality built-in to blacklist domains (via /etc/pihole/blacklist.txt). Instead of the above, you can simply use:

  $ pihole -b trace.svc.ui.com

This will result in the IP address "0.0.0.0" being returned (with a TTL of two seconds) for any manually blacklisted hostnames (the same way that PiHole normally responds to queries for blocked domains) although, personally, I still prefer NXDOMAIN.