Yeah I have tossed around the idea a bunch for my new rig. As woody said the weight of the added cylinders and fluid would be a lot. Plus you would need another pump in addition to steering.
I think mounting the cylinders would take some doing just because of packaging concerns. Beauty of coilovers is their small size around the wheel. I would look at mounting the upper coilovers to a bellcrank that is controlled by the cylinder. This would allow the coilovers to still allow dampening, and have the cylinder just adjust height.
I do think you have to look at what you want to achieve with the hydraulics. Being able to adjust all corners would be nice, but I think priorities would be to dive th front in climbs, and lift the body if high centered. For me I have decided to use the winch to snug the front end down, and air bags to raise the body. Since I want to run low CG I will place ride height as low as possible and use the air bags to raise only in highcenter situations.