Manipulation most likely would effect the facet joint above, the L4-L5, helpful if painful and restricted. The sacralization can be complete bony union or partial in which case there is a probable fibrous union. In time, the disc and joints above and below may achieve greater degenerative changes because of greater mechanical loads. I think of sacralization as being fairly stable. Exercise guidelines would be consistent with stability, control, timing, lenght, strength, endurance, not too different than treating other lumbar syndromes. Education in proper body mechaics seems morally appropriate, even if there is little support from the literature. Screening proximal structures such as hip, pelvis, SI,etc., makes sense and distal structures as well. Your good common sense will serve you well with this client.