Q: With 8-bit alpha, is it possible with the current NWN2 engine to use the alpha to make a surface translucent (as in 50% opacity with an alpha pixel value of 50%)?
A: There are two ways to make a surface translucent.
One way would be to make a special effect that edits the alpha. In the VFX editor this would be called a "game model effect". Then you'd have to assign the effect to the object via the toolset, or script.
The second way would be to export the model that you want the translucent surface on as a "glow object". However this would cause the object to be additive as well.