Besides being able to verify that adding/removing from memcache worked, it helped figuring out how to retreive the stored items and, more importantly, the cache keys.
Since ZMCache supports grouping of entries the only way to handling this is to iterate over all entries and compare prefixes.
Initially I implemented some internal tracking. However, I wasn’t sure whether that would stay in sync over time. Iterating over entries might not be the most performant operation, but considering that this is ever going to happen from within the admin interface of ZenMagick this should be fine.