About E. viminalis, it is one of the eucalypt species with a widespread habitat in Australia (and Tasmania), and this means there are "races" living in different conditions (some alpine, some almost Mediterranean). Some seem to be cold hardier than others, but still, even it being quite a cold hardy type of eucalypt, it reaches its limits around -12ºC (so lightly less hardy than the other species we mention). However, if thinking as a dieback species, it being a good coppicer (as the related E. dalrympleana), the results could be similar to the others we mentioned. Some Snow Gums can also be very cold hardy, but they have limits too, and they are slower growing and slower coppicing in general.

Maybe a way you could try is allowing the small seedlings to develop as big potted plants for 1 or two winters, and then plant them out if you find it appropriate. This way it is possible that lignotubers (where Eucalyptus normally regrow from) get some development, and roots too, hence giving some more chances for later regrowth from coppice outdoors. As the plants are going to be "pruned by frosts", the risk of unestable big trees is nil, so even rootbound plants could work. They being potted, you could move them from outdoors to some type of cold frame or glasshouse with as much light as possible for the winters. Then the main difficulty might have to do with light levels. If the plant was fully active (for instance, with nice temperatures indoors) it would try to keep growing, and would like to have light, something that can be difficult at high latitudes . However, in a cold frame or unheated glasshouse that kept temperature around 0ºC or a bit higher, or at least would avoid direct chilly wind & frost they would be "in the fridge" ("almost asleep") until spring.

Small plants can be quite a bit more fragile and the typical "limit temperatures" you can see quoted in literature are normally for older trees (past the first winter).