From my perspective as a cold weather wuss, I'd agree that riding in 15F is "boundary case". (Actually, for me, it's a "no frickin' way case"!)
That said, for those who DO want to play at the boundaries, a solution is calling out.
I remember many years ago I owned a humpback Volvo PV544 Sport, a gem of a car that, originating in Sweden where the winters are severe and long, had a great feature: There was a long ball chain that threaded through a keyhole opening in the bottom of the sheet metal dash.
When very cold, the driver could pull the chain through the large portion of the hole and then lock it in the smaller portion. This would raise a window shade (really!) up over the front of the radiator to block incoming air, so the engine would warm up much faster. It worked very well, but required the driver to be vigilant and lower the blind to prevent boiling away the coolant and cooking the engine. (This was back when vehicle manufacturers actually expected their customers to think and to be responsible.)http://www.ipdusa.com/uploads/images_blog/181.jpg
The Empulse battery case design understandably favors hot weather and is efficient at cooling the pack. This is a good thing! But wouldn't it be possible to design a temporary measure to lessen the cooling efficiency for those who wish to ride in very cold conditions?
Now wait: I'm not suggesting window shades on either side of the bike, but perhaps something like add-on panels to block the wind stream's access to the battery pack. The top panels would rough out at 4.5 x 16 inches and the bottom panels at 4.5 x 14 inches. An threaded button-head rod with Nylok nuts (or a skewer with a quick-release lever) could run through the slot in the battery pack and clamp left and ride panels to each other, with some plastic projections molded into the panels to align them into the other slots of between the batteries. So, 4 panels, 2 rods, and 2 nuts and you'd be good for the winter season.
Of course some testing would be needed to see at what ambient temperatures and under what riding conditions the panels would have to be removed. Maybe even a sensor to create a readout on the dash telling the rider that it's time to remove the panels (or slide open some vents?) and not fry the battery pack, unlike Volvo which relied on the driver to watch the rapidly climbing engine temperature gauge!
I'm sure there's some more elegant way to defeat the efficient cooling of the pack, but that's what we have Brammo designers for!