Duncan, What serves as the spar web ( transferring the loads between the top cap and the bottom cap)? If I'm picturing this right, you'd have just the foam ribs and an indirect path through the closures at LE and TE. It is the spar web (which joins the top spar cap, which is normally in compression, to the bottom spar cap, which is normally in tension) that gives the spar stiffness. Without this, you've just got very flexible top and bottom caps. Also, the only thing to keep them from buckling (under compression) is 1mm ply to hold them straight. Maybe that's enough, I don't know.