1.12: Cannot send data to client before connection rejected bc of missing registry entries.

Hey there, I'm the dev for OpenTerrainGenerator, I'm working on updating to 1.12. I've also created an issue on github but a fellow dev suggested it may be better to post this here. I'll try to keep this short:

With OTG biomes can be created and customised by users, this means that every world can have any number of biomes with different names and settings. As a consequence biomes aren't known at app start but are loaded and registered when a world/dimension is created (and unregistered when the world/dimension is unloaded). This also means that when a client connects to the server the server sends the biome data to the client, which registers the biomes and then loads the world. For previous versions of Forge it was possible to use FMLNetworkEvent.ServerConnectionFromClientEvent to send the biome data to the client before the player joined the world. However, with 1.12 the connection is rejected when Forge detects that biomes are missing from the registry, resulting in a "Connection lost, Fatally missing blocks and items." error screen. This appears to happen before any events are fired that I can hook into (such as FMLNetworkEvent.ServerConnectionFromClientEvent) so I don't see any way to send the biome data to the client before Forge rejects the connection.

Is there an event I can use to send the biome data before Forge rejects the connection? Thanks!