Although the Framesaver would prevent rust, isn't it better to have a way for the condensation to exit? Couldn't you be left with water-logged tubes or would the moisture just "un-condense" (and the cycle repeat ad infinitum)?
The idea is that the Framesaver is coating everything OK, so you just want to keep the rain out. Even at 30 degrees saturated air's only got 0.3g/l of water. An average seatstay volume might be 250ml, so you're only looking at less than 0.1g of water trapped.
If the internal coating isn't 100%, the Framesaver contains a volatile rust inhibitor, so there might be a little rust, but it also might use up any available oxygen so no more corrosion can occur. No available oxygen, no rust, however much condensation there is. If the tubes are open the cycle is constantly refreshed with air and water via the breather holes and the rust inhibitor is driven off.
I suppose the tubes would ideally be sealed on a cold very low humidity day, but I think that's better than sucking all sorts of water and dirt into the tubes each wet ride, puddle or river crossing.
Some open tubes can drain under gravity to a BB shell drainer hole (DT, TT and CS) if it's a lugged frame. However lugless frames, welded frames, and most head tube joints and seatstays only have small breather holes at the joints, so moisture can't drain efficiently and just pools up inside the joints.
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse