I've added rowanberries to the must with good results. If you use apple juice I would recommend putting the rowanberries in the freezer a couple of times (freeze, thaw, freeze, thaw) to mellow the acid before mushing up the berries and adding them to the juice. The freeze-thaw trick is...
Maybe a late answer but I tried it "by necessity". My must got left in jerricans over a business trip and were almost spherical on my return. So I decided that the best action was to go ahead and pour it into a brewing contsiner and let it brew. That batch turned out better than my "proper...