I haven't tried this but it may work.If there is a command for jump you can try putting a modifier macro on it. Something like this.

(I know this isn't the command but it may work if you can find out what it is)

/jump [noflying]


unbind jump in your key bindings and set this macro to spacebar if it works. This should cause you to jump unless you are actualy off the ground. Otherwise just get usd to having a slow and crappy start when flying.