Ok, I've cut out the deck hatches (the way the design intended them), which gives me a lot more access.
The interior has already be sealed in some way - hard to tell what with. In fact, I can now see that there is the typical 1" strips of cloth from gunwhale to gunwhale, about 5" apart. Including one across the aft of the keel box, along each side of the keel box, but not across the front of the mast box.
I'm pretty much certain that that crack emanates from mast heel pressure, so I think I'll also drop (not glue) in a plate of ali or something, to spread the load around the bottom of the mast box. Perhaps pour some thinned epoxy in first to strengthen the inner base/corners of it. Seeing as the hull does look reasonably well strengthened inside, perhaps that's all it really needs.
I've got some Z-Poxy laminating resin which, it seems, can be thinned with meths?