I found a calendar of propagation activities which seems very helpful, and it says that hardwood cuttings can be taken and started for deciduous trees throughout winter. I imagine those cuttings would freeze and be destroyed if left on their own poked into some soil (if the soil is even unfrozen enough to receive them). That makes me think those hardwood cuttings should be grown in a somewhat protected space, either kept in cool dark conditions (in a garage or maybe outdoors with a thick topping of mulch) or nursed to life under a light, with a heat mat, etc.
Any clarification about the above would be appreciated, but my main question is this: can propagated trees that would normally be dormant over winter be started out indoors? If not, why/what bad outcome would they eventually reach?
Example: a bunch of seeds or hardwood cuttings of deciduous trees are planted in air-pruning pots or beds and grown indoors over winter. Around/after last frost in the spring, they're either planted into a larger pot/raised bed to be transplanted or heeled in after 1 year; or planted into permanent beds to grow for 2-3 years before transplanting; or planted into their permanent location to live out their life. For every winter after the one they're started in, they'll be outdoors in the ground exposed to zone 4/5 conditions.
Will baby trees grown this way be crippled by having grown in nursed conditions the winter they're started and then all through summer, basically experiencing a much longer growing season than they ever will in the future?