My understanding for the reason that Sally Tompkins & Lucy Otey were given Captain's commissions were simply that it would allow them to requisition medical supplies. Otherwise these ladies had to track down a surgeon/captain to sign the requisition. By designating them Captains it cut out a bit of red tape and allowed the ladies to get their supplies.

Greg Starbuck

PS- I'd be interested in corresponding with any of the Otey descendents in this thread.