I don't really have a sensible answer, but here's a few related, random thoughts:
I used a high-spec pair of Accuride slides for part of my screen masking system and while they don't actually bind, they sometimes don't slide 100% in unison - probably not an issue for you as the weight of a projector would ensure everything dropped into the right place. Also, all the slides I've seen could be trimmed and/or adapted somewhat, so you could probably accommodate the slightly too long model you mentioned.
The lab jack you linked to looks promising - if the thread quality was semi-decent I'd be tempted to try to drive this using an electric motor, rather than retro-fitting linear actuators.
Another type of construct to consider would be to utilize a rectangular panel that is hinged on the edge furthest away from the screen and have the projector mounted directly to this at an angle that allows it to be parallel to the floor when the panel is 'dropped' - this gives good stability (by using a solid rear hinge) and allows for multiple options to power the raising/lowering (linear actuators, motor driven pulleys, worm drive, etc.).
It would be prudent, in any design you settle on, to include some form of emergency access or release - you don't want to have to dismantle the ceiling because a cable came loose and jammed the mechanism in the closed position!
There are commercial units that would do what you're looking for, but they all have pretty unattractive price points. One of the cheapest I've seen is this: http://www.mountdirect.com/EPL_Electric_Projector_Lift_p/epl.htm
at about a grand. [Edit: Just re-checked the dimensions, and even this one would be too big! Duh] I use an Auton lift that could be adapted for a drop-down install, but it's way too bulky and definitely not a bargain!
Anyway, looks like a cool project!