It might be possible with reflection, but I spent an hour and a half trying to make it work and couldn’t.
You need to modify 2 locked registries and set their default values AND register your new air block without Forge realising what your doing and marking it as an override. You have to somehow pretend that the mod registering the block is FML/null. I managed to get it almost to work (got it all the way to the resync of the registries) but gave up.
I achieved it in the way we don’t talk about, and air renders perfectly now ?