I am in 6a, and attempted to grow Yuzu Ichandrin (on Flying Dragon rootstock - available from One Green World) outside last winter, and it failed. It was a cold winter in my region (about -13F on the end, plus wind). I mulched mine heavily, but also made what I now consider a big mistake. A drain pipe from the roof
led to the same bed the citrus was in, meaning that there were times where the
roots were not just cold surface soil temps, but actually waterlogged and frozen - which I think it what really sealed this thing's fate.
I also didn't give it my absolute best microclimate, and I only do pretty half-assed insulation (ie. throw a big pile of
straw on top). I'm trying to push the limits of my zone a bit, but not if it takes a large time investment, which I don't have right now. I lost pineapple guava this winter (not surprising), but my Chicago Hardy Fig made it. Some frost-kill on the branches to be sure, but that was true of many things this winter.
Yuzu Ichandrin is your best bet, as far as my research goes. I'd like to try again with an older specimen in a better place. I've got one in a pot to play with and clone from. It's said to be similar to lemon in taste, and used in Japanese cooking.
For a sweet citrus eaten off the tree - I'd agree that either a
solar green house or a potted indoor/outdoor plant is the best way to go.
I'm excited to see what Paul does for his lemon tree outdoors in Montana - that would almost certainly work for Yuzu Ichandrin, because it's even more cold hardy.