ZF did not make the 722.6 transmission. Mercedes' automatics are made exclusively in-house. BMW uses ZF, and I feel that ZF units are lower quality than MB units. ZF designs included lots of units where the front pump did not run at idle, causing heat build up in traffic and premature failure. BMW did not cover those repairs.
However, you are right in that Mercedes will not cover out of warranty repairs that are related to long service intervals. If your 722.6 tranny fails past the warranty period, you're SOL regardless of the schedule of maintenance you follow.
Mercedes has specidfied "fill for life" meaning the life of your average lease...
1998 C230 "Black Betty"