To be honest I don't know much about Kombucha or had much personal experience other than occasionally trying some of the bottled commercial ones that come in a cylindrical shaped bottle. There are all kinds of claims about it's benefits, from improved digestion, better skin, stronger immunity even some cancer curing properties etc in addition to being a good probiotic as most cultured food are. I can't see that it would hurt doing it before or after the MC. My only advice is don't put all your expectations into any one thing
I don't believe in one hit wonders and always revert to the good ole common sense of going with eating raw and juicing fresh daily as a good helpful pre fasting diet to get thing going. If you added Kombucha to that it would probably also be beneficial. and post fast to help replenish the beneficial bacteria in your gut.