I'm in zone 5b, north of Chicago, so my climate is only a little different than Montreal.
Ume are a problem here, not because they can't take the cold, they do just fine the early and middle part of the winter. The issue is, they wake up quickly from dormancy once their chill requirement is met. In late January or early February we tend to get a late winter thaw. Usually a few days to a week above freezing. Then this is followed by a cold snap that can be as brutal as our coldest of the winter. Ume will wake up during the later winter thaw, maybe even get a bloom or two out, then the bitter cold returns and temps of -17 C or 0 F will damage or kill the tree. Sometime we even get temperature below -17 C or 0 F after this late winter thaw. If you can keep the Ume above 23 or 25 F or above -5 C after the late winter thaw the ume should be fine. You will get blooms and normal amount of vegetative growth in spring.
So your protected storage will work fine. But ume can not be grown in the ground without occasional losses of flowers or sometimes vegetative freeze damage in our climates.
For a similar, flowering before leaves appear tree, that will do well in the ground or in a pot without temperature protection in Montreal look into the northern or cold adapted seed grown semi-wild forms of Prunus armeniaca, and Prunus manchuriana the culinary apricot. Cultivars selected and grafted for fruit production often were selected from stock from warmer parts of their range, but the ones from seed, selected for planting as windbreaks and wildlife food are winter hardy into the warmer parts of zone 3.
Another choice is the North American wild plum - Prunus americana - also flowers before leaves appear. Again avoid grafted trees, wild stock will work better.
In the apple family, Amelanchier species, any of them, will flower before leaves appear and do well in bonsai pots. There are Amelanchier species, 3 or 4 native to Canada. I like the hybrid from nurseries, Amelanchier x grandiflora as it tends to sucker less and will stay single trunk tree if you reduce it to a single trunk tree. This is the most tree like of the Amelanchier, the rest are shrubs, and will produce suckers occasionally. Avoid Amelanchier stolonifera, as its name suggests, this one has a strong tendency to produce suckers. All of the Amelanchier are very, very winter hardy, most through all of zone 4, a few species are hardy into zone 3.
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