Many thanks @JendaVodka , I agree your suggested solution is the best way to make this change.
However, the documentation isn’t quite clear how to specify the ssh key file. If the full path is used, the installed errors out. The configuration script really is a bit obscure, but it does work.
# foreman-installer --foreman-proxy-plugin-remote-execution-script-ssh-identity-file id_ed25519_foreman_proxy
2023-03-06 18:11:12 [NOTICE] [root] Loading installer configuration. This will take some time.
2023-03-06 18:11:14 [NOTICE] [root] Running installer with log based terminal output at level NOTICE.
2023-03-06 18:11:14 [NOTICE] [root] Use -l to set the terminal output log level to ERROR, WARN, NOTICE, INFO, or DEBUG. See --full-help for definitions.
2023-03-06 18:11:19 [NOTICE] [configure] Starting system configuration.
2023-03-06 18:11:26 [NOTICE] [configure] 250 configuration steps out of 1457 steps complete.
2023-03-06 18:11:28 [NOTICE] [configure] 500 configuration steps out of 1459 steps complete.
2023-03-06 18:11:30 [NOTICE] [configure] 750 configuration steps out of 1464 steps complete.
2023-03-06 18:11:31 [NOTICE] [configure] 1000 configuration steps out of 1468 steps complete.
2023-03-06 18:11:48 [NOTICE] [configure] 1250 configuration steps out of 1468 steps complete.
2023-03-06 18:11:59 [NOTICE] [configure] System configuration has finished.
Executing: foreman-rake upgrade:run
Upgrade Step 1/2: katello:correct_repositories. This may take a long while.
Processing Repository 1/110: centos7-x86_64-extras (1)
-- x --- snip --- x --
Processing Repository 109/110: Rocky Linux 9 - Extras (x86) (537)
Processing Repository 110/110: epel9 (538)
Upgrade Step 2/2: katello:clean_backend_objects. This may take a long while.
0 orphaned consumer id(s) found in candlepin.
Candlepin orphaned consumers: 
* Foreman is running at https://foreman.my.org
* To install an additional Foreman proxy on separate machine continue by running:
foreman-proxy-certs-generate --foreman-proxy-fqdn "$FOREMAN_PROXY" --certs-tar "/root/$FOREMAN_PROXY-certs.tar"
* Foreman Proxy is running at https://foreman.my.org:9090
The full log is at /var/log/foreman-installer/katello.log