Beatsaber anons we should be good to go now. If I hadn't gotten frustrated last time we might have been able to play as MP is in fact working just fine. The only issue is that custom avatars hasn't been updated on mod assistant as being compatible with 1.5 so you need to install the mod manually. 
So the steps are to run your updated game once. Then run modassistant for 1.5 and grab everything including mapping extensions. Next, get the MP mod from https://github.com/superrob/BeatSaberMultiplayer/releases and merge. Finally, find the custom avatars mod from https://beatmods.com/uploads/5d75ebe8be34f5432f209f20/universal/Custom%20Avatars-4.7.6.zip and merge that as well and run ipa.exe. 
It's more steps than what I'd like but it works. If you want a single zip of stuff to install like what other anons do I can put that together, just let me know.

May host a makeup lobby Tuesday if there's interest.