Wow, about 130 views and nobody had ideas. Ok, I'll answer my own question.
This one (BV Classic Bike Saddle Bag), even in Large size, fits fine (amazon.com/gp/product/B00A3W8FFM).
Amazon.com : BV Bicycle Strap-On Bike Saddle Bag/Seat Bag/Cycling Bag : Sports & Outdoors
It's easy to install under the stock seat, a little tougher when under my replacement Cloud 9 Cruiser Select (barely clears the springs, but it works), but it still fits even with the seatpost within 4" of the bottom, and with both a rear rack and small basket installed. The back side of the bag runs right up to the basket, so you have to loosen the straps for access to the stuff inside, but that makes it a little more secure and harder to steal.
Pretty nice bag, by the way; a Vibrelli pump fits (diagonally) along with a Milton pressure gauge, 4 Gorilla tire levers, Handlebar Jack, Crankbrothers M19 multitool, patch kit etc. - without having to open the expansion pocket at the bottom.