On the
Bansbach page, there are a couple versions. Type B uses a valve that interrupts gas flow between one side of the cylinder to the other. They call this "spring locking", since the gas is compressible, and the cylinder locks with some springiness. The other types put the valve in an oil reservoir, which locks the cylinder rigidly (in one direction or the other or both).
The only concern would be whether the latter types would contravene R9, but I would tend to believe they don't count as "hydraulic devices" in the same way as they currently don't count as "pneumatic devices" (R77 blue box) for the purposes of the safety rules.