Tonight I verified that the rear d-pillars are indeed capable of housing speakers. The 2nd image shows the two screw holes used for mounting.

** Edit. According to the bavarian Soundwerks e70 guide, the d-pillar panel has a "pod" that attaches to the panel. I have no idea what those sheet metal holes are for...
http://www.bavariansoundwerks.com/pr...tage1_0211.pdf