I’ve used Wireguard, and thus a separate monitoring network/VPN, to achieve this.
…actually I’ve also used SSH reverse tunnels and autossh to achieve the same too, but I’d recommend the Wireguard solution if it is feasible to go this route.
The slave connects to the master to set up the Wireguard connection - with PersistantKeepalive to keep the connection up despite NAT etc - and then you configure the Wireguard IP of the slave in the Distributed Monitoring setup on the Master.
…it’s not exactly what you’re asking, and Dirks suggestion might be nicer depending on your setup. Regularly imported dumps are technically very easy to handle - this is the “passive” way, and you’ll just have to make sure you get alerts if data gets stale (slave is not reporting back). The Wireguard solution continues the “active monitoring” approach… 