The standard dormancy temperature in research is below 40F/5C. Many species then have a bud chilling requirement, an accumulated number of hours below 40F/5C, before dormancy can be broken. I think acer palmatum/shirasawanum/japonicum do as well, but I don't know what the hours are. Acer palmatum/shirasawanum seeds require something like 6 weeks or 1000 hours below 40F/5C to germinate. It would seem sensible to expect the bud chill requirement requirement to be similar, even though this need not be the case.
All temperate trees switch their growth mode with the passage of the summer solstice. Before the solstice, cambium cell division rates accelerate and shortly thereafter decline, ultimately ceasing when temperatures drop enough to affect dormancy. In my climate, acers are in the accelerating cambium cell division mode for 5 months, at most. The safe thing to do would be to emulate mother nature with, say, 6 months of 18 hours days and 6 months of 8 hour days with a 6 week period of cold, but it is an interesting question: how do maples behave with long days and low temperature, for example. Of course, if the tree is leafless it likely doesn't matter what the day length is.
I guess that you want humidity in the range 50% <= rH <= 80%. Fungal problems are synonymous with high humidity and still air. I am guessing that keeping it below 80% will prevent this (you could spray periodically with a solution of 2 tablespoons 3% hydrogen peroxide from the grocery/pharmacy in a quart of water to address fungal issues that arise). The rH in my location rarely falls below 50% and these species thrive. It does occasionally dip to as low as 20% for short periods without causing any ill effects. The combined effects of full sun (OM: 100k lumen/m^2)and mild winds/breezes do cause leaf damage, but it is hard for me to conceive of how this would occur indoors.
EDIT: IMHO, 3% peroxide could be used to sterilize the grow box from time to time to limit the possibility of fungal problems. Perhaps it is advisable to sterilize the grow box before introducing the maple. The diluted spray I suggested could be applied to the entire tree and as a soil drench prior to introducing the maple to the grow box.
