After finishing up the sanding & grinding, I created a backing piece to glue onto the inside of my gap. I molded cardboard and covered it with plastic packing tape to match the interior profile of the hull corner. I laid two pieces of wet-out 6oz fiber cloth onto the mold, and peeled it away when cured. The backing piece glued almost perfectly into the hull's interior with some ketchup-y resin+colloidal silica. I filled the low spots above the backing piece with filler & now I'm ready to start laying glass inside and out--hopefully three more layers inside and at least three on the outside.
http://www.flickr.com/photos/12713306@N02/6019754000