I have no idea where you would get mercury from a hydraulic system on anything. This story kind of sounds like urban legend, or the crew had no idea what they were taking apart.
Having worked in SFX, I guess it was probably from a Chapman (a sort of crane balanced seesaw big enough to hang cameramen and cameras from) - at one time (presumably no longer) I believe they used a big reservoir of mercury to achieve fine balance, to the extent you could move swing them around manually.
Amusing side note: They had a locking lever when obviously had to be engaged when the cameraman wasn't on the end. If someone freed the lever with the crane out of balance, the results could be quite catastrophic (source: I could've been killed on one occasion).
I heard the story second-hand on a podcast and obviously confused some of the details, but I think you are right about the piece of equipment they had access to.
