I discussed my problems with my local resin and glass suppliers, and they offered to lend me a vacuum pump they use. I think that should work great. Drill a matrix of holes on the inner skin, use some garden fleece (44's 'shade cloth'?) under the plastic, and pump till no more water
I have dug into some of the areas that showed up wet with the meter, and it is not anywhere near as bad as I feared. The dampness is slight, the foam is fine, and the glass well bonded to it. I found just one area where the glass was mushy, about a square foot, and that was just the top 2-3 mm. Under that, the glass was dry and solid. I guess that was an area where the glass wasn't wetted out properly, and then water
I'm going to do a bit of sailing this summer, then get the boat under cover for the winter, and do a refit
of all deck
fittings, and apply the pump where needed. That should do it.