I think I've figured out the answer myself. The UM506 Universal Module seems to be a simple contact switch, just what I was asking about.
I opened the back of the screens inline controller (which is not attached to the screen at the moment) shorted across the switch soldier points and the controller sprang into action. You can tell what it's doing by the way the LED flashes. So I see no reason this method shouldn't work. Does anyone else?
The upside about using this method is I can leave the original manual controller attached to the screen and still functional.
The downside is it will take three UM506 modules. One each for up, down and stop.
Kudos!
It sounds like you have figured your solution yourself. Trying to keep your screen in as original condition as possible is the most desireable solution.
But why do you need the third "stop" UM506? Is that to stop the screen some where between full up and full down?
At any rate, your test pretty much confirmed a UM506 will work - as long as you do not have to maintain the short across the momentary switch for the full extent of the screen travel time.
When sent an "ON" command the UM506 will do either: 1) provide a momentary closure of about 1 second duration, OR 2) provide and maintain a closure until an "OFF" command is received by the UM506. The UM506 also gives you a beep beep beep every time it is energized, which can be silenced if undesired.
If you decide to go the 120V relay route, this is a cheap relay which works good and can be powered directly from an Appliance Module. The same surplus house has low voltage AC and DC coil relays and also plug in (wall wart) power supplies IF you want to use low voltage relays, but that just complicates the design.
http://www.allelectronics.com/cgi-bin/item/RLY-453/500400/120_VAC_RELAY,_DPDT_12_AMPS_.html