the BMW option for double glazing is S2-something... It was installed on some non-US E53, I even talked to a guy who needed some help with his car, and his car came with double glazing option... He was going to snap a picture of his glass but somehow it fell through the cracks...

the Electrochromatic (as opposed to Photochromatic, when the tint changes under the direct sunlight) is available with double glass pane (construction of buildings) and as a film (auto and construction). the film thing will require some labor, as the film needs to be cut and than "terminated" for the wires, that are to be soldered/glued onto the film and then wired up to power source. That is the reason I am saying, that the moving windows will need to be pulled out from the frame before properly installed.

it seems that this whole thing is doable, the main questions are - how much, and where...