I repotted a Kingsville last spring around the same time as maples were breaking bud, leaves were still bronze, did significant root reduction and bounced back quite well. Likewise for standard boxwood at the same time, also responded well. Anecdotal to be sure with only 2 data points for you!
For your climate, no question about it. Would be fine. Even in my climate I will be re-potting in about 6 weeks and will be done ahead of leaves greening up.
Actually I am a bit surprised that they even bronze in SO CA.
We appear to have a similar climate to CA and boxwood appear to be happy with repotting and root prune any time from mid winter here whether leaves are bronzed or green.