I assume the antler goes hard again once it dries out. Any concerns about rust putting the tang in wet antler?

The antler does return it's hardness. Rust is not a concern. The steel is hot enough that it evaporates quickly and once sealed so no air can reach it, rust needs oxygen to form.

-- There is nothing like the sound of a well tuned hand plane. - https://timetestedtools.net and https://diy.timetestedtools.net