Perhaps a 'sticky' in the "Latest Version" forum clearly stating that MB is still being developed and supported, but author participation is somewhat limited at the current time due to the continuing development, and eventual release, of v3. Perhaps a "READ THIS" subject title. And lock it. You don't need/want comments.
Granted, people who don't read the other notices probably won't read this, but with it being in the 'latest version' section, maybe it will catch their eye.