Use the Mod1 and Mod2 CV inputs with a two axis controller of your choice. ( Not available to those with an Explorer. )
So for instance Doepfer makes a double wheel CV input device.
You can also get this in a version with both wheels unsprung.
Or how about a joystick instead. That would have been nice on any of the other Hydrasynths.
Erica makes a fancy joystick controller, that can do stuff like record up to 8 seconds of motion, and then loop that.
That would be fun on your HS. This one has all sorts of crazy other modes too.