The most effective way to deal with obsolete Tombstones in the short lived cache case seems to be to drop them on the floor en masse... :D
a) have two column families that the application alternates between, modulo time_period
b) truncate and populate the cold one
c) read from the hot one
d) clear snapshots frequently
This avoids the downsides of dealing with Tombstones entirely, with only the cost of increased complexity to manage snapshots. One could (NOT RECOMMENDED) also disable automatic snapshotting on truncate...
PS - apparently in the past this would have resulted in schema CF growing without bound, but that is no longer the case...