It's still open ended right now, but I'm working it out in my head. I've got quite a few other things that I need to code on before getting back to PXO though.
Just getting the server side fully ported/portable is still my primary work for PXO. The client/host side upgrade to handle all of my proposed multi changes (dynamic server client version bumping, automatic checks/testing for compatibility and capability, etc) will come after I get all of the stuff fully ported cross-platform. PXO is a ****-load easier to make big-endian compatible than FS2NetD is, but it's still got its quirks, and the server side is an almost 100% from-scratch rewrite.
I want to make mods parts of the compatibility/capability checking. The host will advertise what capabilities it needs (ie, mission/mod in use, number of ships/weapons that need to be supported), the host version will be dynamically bumped based on those capabilities. Each capability will be put together to form a particular version string. When the client gets this particular version string (in the broadcast packet) it will see if it also has those abilities. If so then the host is made available. Per some discussions with multi users previously, all hosts will be listed if a client supports the dynamic versioning, but if the client doesn't support one or more capabilities for that host then it will be greyed out and unusable.
The PXO server itself won't have much, if anything, to do with this. The PXO server is only going to offer tracker capabilities for user sign-in/stats, chat, and game tracking. The mod features will be totally on the FS2_Open side so that we don't have to keep upgrading the PXO server with new capabilities, just get it working and keep it working.