Thanks for the replies guys.
I am pretty sure that you can, but I cannot find any info on the net. The Tarifplan (that blue bordered poster in all the u and s bahn stations) shows a few exclusively DB routes (to Aßling for example) and loads of RVO bus stuff within its zones: I am guessing they wouldn't do that unless they were covered.
I don't know whether it makes monetary sense for DB, but there are quite a few things they are required by the goverment to operate. I will pop along to the
MVV counter on my way home, if nowt else comes up here.