I too have considered foaming the hull of my 1969 20.
My deck has held up surprisingly well over the years but when I eventually replace it would seem like the logical time to foam. My biggest concern is waterlogging, so many times I've heard complaints of "wet foam" and the excess weight that it brings with it. I'd assume it would make sense to place conduit along the keel to allow water drainage from bow to stern and compartmentally, foam over that. Trayder in his redo has some good examples of this if I recall.
I've even seen someone somewhere mention filling the hull with airtight, sealed, empty 2 liter plastic bottles!
Maybe crazy, maybe ingenious.