Cannot do.
Everyone would need to download the plane mod, and anything like this has been a definite no from andrew.

 

Dont suggest mods. Suggest plugins. Mods would require EVERY single user to modify their MC Clients. Sorry. Plugins add tons of really cool things without making people modify their minecraft clients. The difference is, Plugins take use of old MC items, such as boats, and transform them into cool new items, like planes, without adding a new block. They still look like boats, but they can fly because its a plugin. Mods add new items, which can only be seen in MP if the user has modified there minecraft client with the mod.