If you live in a freezing climate you either want to displace the water or drain it. A garboard plug has its own liabilities. I'd prefer the builders didn't build inaccessible cavities into boats as generally bad practice. Mine has the same hollow keel that many have, I haven't filled it yet but I have dried it out and sealed the ingress.But why put foam of any sort in? I can think of no benefit and lots of challenges.