We talked about beam splitters earlier this year, I never got to starting making one, but now when I've tested the Sony HMZ as a FPV HMD I'm more motivated to get SBS Stereo images from my Quadrocopters little camera
If the extra weight is centered on the copter it can take a payload of 100-200 grams, at 200 grams the battery time is below 50% of original so not very practical. So around 100 grams, but thats if the weight is centered, the camera on the copter is at the tip of the nose (Pic showing the camera)

I bet the weight of the beam splitter cant be many grams before the copter gets unstable, you can maybe use a counter weight at the aft, but still I really need to find light weight stuff.
My friend as a CNC rigg so I can create light weight mounting for the mirrors using aluminum or some composite material, but were can I find descent quality mirrors that are light weight (And easy to cut btw)?
Optical hardware experts like Palmer probably have a good answer

Thansk guys