J.G.
I am a pediatric PT and to be honest I had never heard of cranial-sacral therapy to treat plagiocephaly but having just read a bunch of articleson it I would definately give it a try. Your difficulty might be in getting insurance to cover it. If you do go that route I would definately be interested in hearing the results.
As the pp said at this age there is less movement of the skull so the evening out is more difficult. You could go get a second opinion for a doc band at cranial technologies.