If you want hydraulics for your lowrider bike you will probably get suspension front forks, long ones will be a very popular choice, these will bring you bike lower for even more coolness and gives you a more easy-rider look.
Get the longest forks that are available and they should have a lot of travel on the legs, this will give a more comfortable ride, more of a glide than a ride, really!
As much chrome as you can get is of course the most important thing with any item for your lowrider.
To be different from all the other guys you could get a hydraulic seat-pin, this goes between the saddle and the frame and give you suspension for your seat and a bit of damping to any bumps in the road for a smooth ride.
A full suspension lowrider may have been made, sounds a great idea, but I’ve not seen one yet, keep a look out because I’m sure some one will make one, one day, for sure it will be very shinny and very low.