Perhaps a little off-topic here -
In my belief, there are other citrus that are better candidates for hybridization than Poncirus, because, while Poncirus is very cold-hardy, it always seems to confer its bitterness and less-than-pleasant taste to its hybrids.
For trying to breed cold-hardy hybrids, the better candidates to use would be Yuzu, Ichang papeda, Ichang lemon (kind of rare), and possibly to a lesser extent C. taiwanica. (Kumquat hybrids also exist but I wouldn't describe them as extremely cold tolerant)
Despite oversimplified presumptions, Yuzu and Ichang lemon are not descendants from the Ichang papeda, although they very likely did come from some unknown papeda that, although no longer surviving, was probably very similar to Ichang papeda. I would view Yuzu (Yuzu is the Japanese name) and Ichang lemons as siblings. Not that they necessarily came from the same particular parent plant of course, but they both are known to have originated from the Southern highlands of China, and very likely from the same type of papeda, so they retain the cold-hardy genes from this papeda ancestor. (In fact I somewhat suspect Ichang papeda may not truly be pure papeda but may be a naturalized hybrid between Yuzu and the original papeda species, but this speculation would be controversial and not really well supported)
I believe that even if hybrids between these citrus do not immediately manifest their cold-hardiness, it is possible second generation offspring may, and this is worthwhile to investigate. (For example, even if a Yuzu x pomelo hybrid does not possess any remarkable cold tolerance, further F2 hybridizing with a mandarin may lead to something that does have remarkable cold tolerance, more than the mandarin parent)