Justthinking about the false-cable twist in the InKnitters magazine. I think it could be used to make a reversible false cable.
The basic twist as I recall it is to knit across the cable, then put all the cable stitches onto a dpn and twist the dpn, then knit off the stitches again. Then if it's wide you have to knit back and forth again to give it enough room to twist.
If you had the previous stitches in stockinette, and the following stitches in reverse stockinette, and did the first half of the cable in knit and the second half in purl, I think the result might be fully reversible. I'm not sure how it would look - haven't tested it - but it might work.
=Tamar