This post comes up when searching for a hydraulic lift solution for dinette to bed solutions, so I'm posting here.
I was thinking about using a floor jack from Harbor Freight. I would obviously need to do some engineering/building as far as getting the lift attached to a table top, as well as creating the support from the lifting surface to the bottom of the table, but there are several floor jacks with a 15" lift, so it should work as far as the amount of travel goes. Not to mention they are designed to hold 3 tons, so there shouldn't be any issue with whatever you might set on the table!
I also saw this video