The next suspension innovation for the Mini was the Hydrolastic system introduced to the saloon models in 1964. Hydrolastic suspension involved similar rubber cones to the “Dry” set-up, but they formed a displacer unit with an integral diaphragm and damper valves. They were filled with a water and antifreeze mixture and interconnected front to rear via pipes under the car. This gave the car a rising and falling motion rather than the sudden pitching of the rubber cone system. Alex Moulton takes up the story once more...
The Morris 1100 was introduced in 1962 with the first Hydrolastic installation. It was a great success, and Ford in particular were very upset by it. Dunlop and Moulton Developments took some time to make a unit small enough to fit in a Mini, but it was done by the early 1960s. The majority of the Mini’s racing and rallying success was achieved with Hydrolastic cars. The whole Mini racing thing was started by John Cooper. We never thought about racing during the design phase; we were worried about safety, particularly when the car was overloaded with lots of students aboard or such like.”
The final phase of the Moulton suspension system evolution was the Hydragas system. In this later version of the interconnection principle, the rubber cone was replaced by using a gas under pressure. Hydragas was introduced with the unloved Austin Allegro in the 1970s and is still in use on roads today fitted to the MGF.

Found something

So Hydrogas must be Hydrolastic with gas instead of fluid. So you could convert Hyrolastic to gas with the right displacers and tubing.