Browsing the literature, it seems the industry has started moving away from highly nucellar rootstocks, which is great news to those of us with breeding aspirations. Most of the new HLB resistant rootstocks have very high rates of zygotic seedlings, at least according to citrusrootstocks.org and the papers I found for the newer USDA releases.
While I haven't found specific cold hardiness data, just looking at perigee, two of the new ones, US-1279, Changsha x poncirus, and SuperSour 2, poncirus x (c. aurantium x c. ichangensis) ought to be very cold hardy. US-1279 is listed as 100% zygotic. Supersour 2 has not been determined yet, but one of the stated goals of the SuperSour program is to breed highly zygotic rootstocks, so it's likely this release will be very good for breeding as well.
And since these are USDA released rootstock plants, they should already have great disease residence, including some HLB resistance, vigor, and productivity. Assuming the hardiness is as good as it ought to be given their parents, I'm thinking these two might make great alternatives to the other poncirus and poncirus x varieties that are currently out there.