I have found something weird using P6 SR3.  Mine also wont do translucency using the single sided square. However, I then tried the normal square prop, and translucency worked, but unfortunately gave the double-sided striping effect. When I activated Normals_Forward, the translucency again stopped working.

The picture show the normal square prop with translucency. I am using a single spotlight, set behind Woody the Mannequin, pointed at the camera. Ray-traced and shadow-mapped shadows give the same effect. 

Hmm, this is a rather annoying facet of SR3. I wonder if the translucency node works correctly in P7?