We have a permanent MP server, which is always on - at least, it should be. Plane distance is 500nm. It's currently not set up to track hours, and it isn't connected with the main FG multiplayer servers. I may connect it, I'm still debating it.
To connect: --multiplay=out,10,mpserver.opredflag.com,5000 --multiplay=in,10,,5001
I advise against connecting it to the other mp servers, at least during important activities such as events, because disconnection insulates the server from various problematic possibilities involving the main server network. For example, someone spawning a 707 on the main network will not crash people on an isolated server.
The 500nm (or longer in the future if necessary?) distance has great potential. Apart from the obvious (AWACS) use, it may also enable various other types of activity such as ELINT and the operation of drones over realistic ranges.