Possibility Make sure your request is even possible. Bukkit plugins can, for the most part, only modify what the server can do. Here is a list of things you cannot do with a plugin: Create a new block or item.
It doesn't support Spout and isn't quite a plane, but the THX Helicopter Mod is available for Bukkit (I run it on my server). Note that it requires everyone who connects to your server have the mod installed. Having actual airplanes would be very cool, but sadly Flan's Plane Mod isn't SMP yet so I think helicopters are the best we're going to see for 1.2.5.
Again. Spout. Spout client interacts with the server mods and make it visible to the player, Spout Craft makes the mods avalible for multilayer use. Not going to say this again.
Unfortunately most mods are not compatible with Spout, THX Helicopter Mod included. To use it you have to manually install the mod on your client, and use the ported mod from MC Port Central on the server; it won't work with Spout. Maybe in the future this situation will improve and Spout will be more compatible but for now in 1.2.5 it's basically either you can choose Spout or choose mods. IMHO there is a lot better selection of mods available so that's what I chose but YMMV.
Exactly that's what the FML/CraftBukkit port of THX Helicopter uses. Full source here (open source MIT license) if anyone's curious how it works: https://github.com/mushroomhostage/minecraft_mod_thx/tree/v018_dev/bukkit
