Just wanted post an update.
Based on the inputs I have received in this thread, I have procured and begun testing on KWZI IG Blue Black and Sailor Kiwa Guro inks. Based on initial findings, I believe that both inks fully meet the requirements I defined in the original post (water resistance, etc.)
KWZI IG Blue Black is a great performer. It's much "wetter" and lubricious than R&K Salix, goes down a much darker blue than Salix, and cures within minutes to a blue so dark that it is almost indistinguishable from black on high-quality papers (e.g. Maruman Mnemosyne, Kakuyo, Clairefontaine, Rhodia, etc.). Strangely, it seems to take longer to cure to its fully-darkened state on my lower-quality papers. My initial water-resistance testing is very promising, but I have yet to test water-test it after having cured for 24-72 hours. I have this ink loaded in my test-bed Platinum 3776 Century with Cosu nib, and will evaluate its performance during my daily signatory work over the next work week. More to follow.
Sailor Kiwa Guro is... amazing. I know that in the OP I was inquiring about Iron Gall inks, but the recommendation for this nano-pigmented ink really appealed to me. I have been using this ink at work for the past couple of days in two Pilot pens (a Fine Kakuno and a Medium Lucina). It is incredibly dark... there may be blacker blacks, but this is plenty black enough for my needs. It is an extremely well-behaved ink, with a really lubricious feel. Water resistance has been utterly fantastic in several tests. It performs well both in general use and for annotating marginalia on the very cheap paper we have at work. It is not problematic in the least in terms of bleed, feather, spread, or echo. Again, more to follow as I continue testing.
It's too soon to say that I've found my holy grail inks, but these inks are really impressive. I'll continue posting updates as I learn more. Thanks to all of you for your inputs and recommendations!