With our Hobie 18, like you said, we tied the tiller crossbar down securely to the rear beam, and that held the rudders in the up position. With our Nacra 6.0, we had a skinny piece of line with a loop in the end of it for each rudder. Put the loop over the tip of the rudder blade and tied it up somehow to the rear beam so the rudder could not fall down.

I don't know the rules about overhangs. Just that you cannot obscure your trailer lights. And I know if the mast extends beyond a certain length, you are supposed to tie a red flag to the end of it.