Instead of w/kg, why not just a global ranking system and divide people by ranking? No need to even publish everyone's rank, just assign people into Gold, Silver, Bronze, etc. Categories. There are plenty of mathematicians who could design an appropriate ranking system. Sure, you could game the system by purposefully losing, but a big win will increase your ranking quickly and the system should know how good you really are.

Plus, if Zwift is really serious (which they are) about eRacing, then you can easily create a league of only the top 1000 racers (or however many you want) and promote the top riders. "Pro league" or something, and have distinct racing seasons.

If you want people to be able to take a night off, simply have ranked and unranked races.