The purpose of a thru-hull backing plate is to distribute the leveraged loads from operating the seacock and removing hoses. The backing plate needs to be stiff enough to distribute the loads to the full contact area and starboard is just too flexible for that purpose, especialy at larger thru-hulls such as head discharges. Cutting boards lack the strength of even starboard.
Plywood is often used for backing plates, but as SailingDog notes, the plywood should be epoxy saturated once drilled for all of the fastenings. I would suggest using MAS epoxy since thier penetrating epoxy does not produce an amine blush, which with other brands may interfere with the bond between the sealant and the backing plate.
If your hull is cored the coring should be cut back and the core space filled with solid thickened epoxy. Another better solution is to cut away the inner skin and remove the core and then build up solid glass and epoxy, tapering out onto the inner and outer skins.
Which brings me to my favorite backing plate method, which is to simply build up layers epoxy and fiberglass with a tapered edge.