Setup your test virtualmin server as normal including DNS.
This server would have a static IP.
Dont bother with port forwarding as this is only needed if you are going to make this server live on the internet.
In your hosts file, use a mapping like 10.0.0.72 mytestwebsite.com
Now you can connect to the test virtulmain server
You do not need to do anything weird with virtualmin DNS to get it to work on a local network. I think you are over thinking it.
You don’t actually need a DNS server to be able to map an IP to a hostname, this is where your host file or ‘static mappings’/‘split DNS’ comes into it.
I’m not sure if I am overthinking or I just don’t understand. Either is possible or both.
My idea is simple but may not work. I only have to add a DNS1 to router and install dnsmasq on virtualmin machine. I lift my remote from sofa and start virtualmin machine by magic packet and all routing goes to virtualmin machine from any browser on the LAN without changing anything on those computers.
I’ll mention that Virtualmin also supports views in BIND (which is something not a lot of control panels can do, last I checked, though admittedly that was years ago), so it can manage two copies of the zone, on for local clients and one for public clients. But, I got the impression earlier you didn’t want to manage DNS with Virtualmin. But, again, there are many ways to do it.