However I can't use Sage pokemon in team builder because no matter the format chosen, they appear when searched, but are marked as 'Illegal'. I can successfully host a server locally and connect to it with the custom client - both of which have Sage pokemon implemented of course. We've been following this guide on adding custom pokemon, some abilities have been added but are untested.
The readme explains how to use the client to connect to a server.
The readme contains a step by step guide to running the server. It's something like a copy of the original project, so the amount of contributors and commits is also copied and does not reflect what's been done only to the Sage version.