It's a fun idea, but Hydra is the only high level spell that I know of which is absolutely worthless. It looks cool, but as any Balace wizard can tell you, it's rarely worth the pips. The problem is simple: In order to boost a Hydra, you have to boost 3 seperate attacks. Each attack is fairly weak. Since Balance wizards already have elemental damage (in the form of Spectral Blast), we really don't have a use for Hydra very often. Spectral Blast gives more "bang for the buck" ( 4 pips instead of 6 for Hydra) without wasting blades and traps.
So, a "Spirit" Hydra might seem cool, but it really wouldn't be all that helpful. My Death wizard would still use her 6 pips for a Wraith.
i have a grand master in balance and hydra really does stink and i only use it when i cant find the only usfull spell judgment. why? because for it to be strong you need 2 seperate pips for elemental blade and elemental trap, then 1 more regular pip for blade storm, then you can cast your regular balance blade.
if there was a spirit hydra it would just be the same method of using 9 pips for a strong hydra.