I changed out the plywood with 3/4" of AL plate. The material needs to be large enough to cover both of the floors which are (basically) located under the compression post built in to the corner of the head enclosure. I built the plate wide enough to spread the load to within a few inches of the hull. Not to close, you don't want to build in hard points....
I had the cabin sole out from the middle of the head door aft to the companionway steps to permit other repairs so I had full access. If you carefully cut the sole out aft of the head, about a foot, you should have enough room to do a similar repair and then cut to fit and reinstall the original plywood.
One change I would make is provide either a slick bearing surface for the post to "work" on of perhaps add provisions to shim the bottom of the post. Under a hard sail the bulkhead sometimes growls as the boat hull flexes under load. Need pictures?