For anyone reading along who needs the forwarding part today, the [logs.transport.syslog] setting this PR builds on is documented, though on the backup settings page rather than next to the audit log, which isn't where I'd have looked: https://docs.xen-orchestra.com/backups-and-dr/backup-features-and-settings#send-xo-logs-to-an-external-syslog-server
On the design question (a log line through the existing pipeline versus a dedicated syslog transport in the plugin) I don't know the plugin well enough to have an opinion worth reading, so I think it's one for @Team-XO-Backend. The PR has been open since late September without a reviewer, so a look from them would help it along.
I'm also curious about your second question: if others here already ship audit records to a SIEM, saying so would show how many people need this.