It’s really cool to see other groups working on cold hardiness in citrus and the multiple seed sources that have been linked.
There’s enough people here interested in projects like this I feel like achieving a “cold-hardy citrus landrace” is pretty realistic.
Yeah! Here’s to having multiple landraces of cold hardy, delicious citrus! 
Ohhh… wishful thinking here… Citrandarin or citrange x blood orange…
A cold-hardy blood orange would be awesome. 
I don’t know if this has already been mentioned here, but this is a tip from a linkedin post:
“Most hardy citrus hybrids are based on Poncirus trifoliata. This a great hardy plant but it’s fruits can be quite bitter.”
"The molecules which produce the bitter taste are not just present in it’s fruits but also in the leaves.
This means that you can easily select already small seedlings by tast-testing the leafs."
(It was actually a post by @Raphael_Maier)
another solution for rusticity is to work with Citrus ichangensis (Inchang Papeda) which has no bitter poncirine in the fruit, even if it remains unedible.
I have an ichangensis x sinensis full of flower bud right now, I am looking forward and hope to discover the first fruits this year. 
That’s an interesting idea! How hardy is Citrus ichangensis? What makes its fruit inedible?
yes -15°c /5°F ! it’s one of the ancestor of YUZU.
inedible because really not good taste… 
but it is very interesting because crossed with poncirus for example it gives Ichang Papeda ‘Ivia’, a good fruit. Two plants very bad in taste that produces a good fruit !
So when it hybridizes with another citrus it loses its very bitter character, whether the other parent is bitter or sweet. 
That does sound like it has interesting possibilities!
A cold hardy sweet Citrus hybrid is a holy grail of plant breeding! I’m very interested in this topic and have a dream of contributing to it.
The deeper discussions on this are on the “Cold Hardy Citrus” forum on tropicalfruitforum.com.
Growing Citrus from seed. I recommend starting fresh seeds in late winter or spring in moist vermiculite. Once sprouted, they can be planted and grown in bright, warm conditions (warm greenhouse or fluorescent lighting indoors). Expect to fertilize more often than with other plants.
Zone 6 - start by growing Poncirus trifoliata (trifoliate orange), then seek out the Conestoga hybrids and the hardiest of the Poncirus hybrids.
Zone 7 - Try Yuzu and related cultivars and Poncirus hybrids like citrandarins, citrumelos, citrangequats, and citranges.
Zone 8 - The range of what you can try opens up considerably, including kumquats and the hardiest of the mandarins.
If you’re willing to try protection methods, these are extensively discussed at cold hardy citrus discussion groups. Electric lights that provide heat, insulating blankets or plastic layers, water barrels, are helpful but all require time/effort, energy inputs, and infrastructure.