Ha ha - progress!
I was puttering around in the workshop just now, and thought "live on the edge.." so I grabbed my trusty (?) can of Lustrekote clear, and bravely shot a 2" wide band across the tail end of one side of the fuse, laying it on a bit heavier at the very end and tapering off to a mist coat.
Only the very heavy (near sagging) part showed any bubbling, and this settled down again after a few minutes to flash off the solvents. The other areas were absolutely fine. Hooray!
The bare foam is not, predictably, this is just EPS. But the paint finish seems to be sufficient barrier to a light coat of Lustrekote. There are stripes on the bottom of the wing that are masked off bare foam.. I'll have to shoot PU there first I guess.
Again, I am convinced the wing is stiff enough for all but the worst 1/2A hijinx. This is a bird to build light, lighter, then lighter still, and I think an .049 and tank plus some micro gear will deliver on that account. I believe this is a perfect home for the VA MK II on its side, with a Fuji bladder tank (it fits). I'll trace the fuse profile on 1/8" ply, cut a nose plate, then sand off 1/8" from the left nose half. Easier to do than to describe.