So I know currently, mirror mode doesn't support proxies, which is understandable considering OSBuddy/The regular OSRS Client doesn't have proxy support. Perhaps this could be useful to an admin to add Proxy support to mirror in the future.
So normally, you just have to use your own IP or a VPN on OSBuddy, however there is a way around this.
There's an application that can be downloaded which allows you to proxy specific programs, rather than VPN'ing your entire PC.
If you download proxifier, you're able to setup your proxy and use it on specific clients, such as osbuddy.
I know this isn't a lot of info, but perhaps this can help a dev in the advancement of mirror mode.