I'd be ok with that if we also a) make public every time the data is accessed, including who is doing it, and b) require that anybody accessing the data have at least as much data public as they are accessing.

I think the worst problems here are around asymmetries. If I secretly have all your data but you don't know that and have none of mine, there's a power imbalance. But if we have equal amounts of data and know when each is looking at the other, that seems far more reasonable to me.