The compression post is sitting on the cabin sole. I guess there is a structure under the sole that goes to the bottom of the hull on top of the keel. How would you transfer the upwards load of the chainplates to that?
I think the best way to do that is to use the bulkhead. I would replace the beam on the side of the bulkhead opposite the compression post and more or less duplicate the original design. Or I would move them outboard a few inches and use the hull itself. Bolt the chainplates through the hull afer first beefing up the area by glassing, with epoxy and biaxial cloth, a thick pad on the hull inside to spread the load. The pad should probably extend about 18" down from the hull/deck join and a ways fore and aft. This will transfer the loads to a much wider area.
Living aboard in Victoria Harbour