After looking over this thing a few times, I'm beginning to think that the issue of mark-only decks isn't as big as it initially appears, as you have to put a Dragon or something similar on your first slot of the field to get full lockdown potential (which is harder for Gravity, given that it's vanilla creatures are all Cost/attack inefficient and it's useful ones either lack attack or have momentum to ignore this shield entirely.). As a result, the potential imbalance doesn't seem as significant.
I'm considering this Crucible-ready, but I've put it at the bottom of my queue in case people have further comments.