Doug, the downside I see about the foam is that it expands. I have a can of foam on board that I was going to use for the icebox insulation, and have not used because of my concern about expansion. Since our water temps here are 57 degrees year round, and we have a large inverter charger and now a new alternator regulator, I have put that on the back burner, knowing the house bank can keep up with the amp load of the fridge on longer excursions.
"...thinking is that filling the voids would reduce areas for water to travel or pool. That might make it easier to deal with leaks in the future. The foam might also add some stiffness and insulation in areas around the ports, which may help maintain a seal longer."
The concept is to keep the water out, and in that regard the foam will do nothing. Stiffness and insulation is not the issue, waterproofing the port(s) is the only way to keep water out.
Your choice, but I wouldn't use those reasons to use the foam. I would think that the only reason would be for the foam to act as a "base" to put in the epoxy. Once the water gets in, the idea is to keep it limited to the interface of the port and the fiberglass part of the hull so it doesn't get into the voids. Adding foam covered by epoxy or epoxy only at the opening seems the way to go.