Rolls did use Citroen hydraulics but - er - its all rather different. I think they used semi trailing arms as used to good (or bad) effect by Truimph and BMW too name 2 rather than trailing arms used by Citroen - and their primary suspension medium is coil springs. The hydraulics are there - really just to provide the self levelling. They may have fully powered brakes as well - I'm not sure on that one. The pump is driven from the engine with a push rod or something - which may have more than a passing resemblance to a GS - but again I'm not sure.
Its common amongst mechanics repairing old Shadows and presumably Spirits just to abandone the self levelling as its beyond them to understand it. This presumably explains why many a well polished Roller seems to have a tail down stance.