"The hive" is one of the forums i'm on, also "aliens legacy".
As for the mechanism, i have been working on some plans for over a month now, and believe i have a pretty good concept. Using 20mm conduit, 16mm doweling, a 18mm compressed spring and 6mm doweling. Dont have a picture of my plans as they are all just sketches but to sum it up, it's going to be a spring loaded device with a latch at one end which has to be physically moved (with wire or a hand) and will shoot the doweling out of the conduit which will then stop at the end thanks to the 6mm doweling wedged into the top.
The only problems i have faced with this is space. I currently have a "monsterroom" xenomorph full body suit and there is only around 250mm room in between the teeth and the neck where your face goes, hardly enough room to mount a spring loaded device aswell as an alien tongue attached to the doweling.
I am still working on ways around this problem, but for the meantime have gotten my hands on a pretty damn good tongue and will just mount that with velcro on the bottom of the head to allow for quick attachment and removal for different phots, i'll post some shots up soon of my sketches and tongue