DLPC350 to DLP4500 flex cable for 310-Series Mounting System - Concept 2

Is there a routing solution for the Concept 2 system which utilizes two 4x10 BGA interposers?  The alignment cutouts in the Concept 2 design leave very little room for trace routing and the mechanical model calls for a total flex thickness of 0.188 mm (7.5 mils).  This would imply no more than three layers.  We tried a 4 layer design (GND and VCC as two inner planes).  While the design was successfully routed and worked electrically, the cable is rather stiff.  I am curious if anyone has resorted to embedding some traces (VCC?) within the inner ground plane without compromising the 50 ohm impedance requirement on the bus and clock signals.