RE: New ballast system
The bag is in the dry compartment, and the change in pressure is very small. Thats the "source" of air to surface.
I will install it in a tight compartment (I don't have a sub yet) and make it work for you.
It doesn't have a valve, it has a "virtual valve": the narrow tube lets air escape or enter but prevents water pass through it (maybe some drops but it doesn't count).
Combining all the aspects of ballast systems, safety, reliability, effectiveness and cheap, it should be nice choice.
Regards.