util.datamanager has a callback functionality that allows overriding or canceling store operations.
Since the introduction of storagemanager, this feature may not work depending on which storage method is set, since only internal storage uses datamanager directly.
This should be deprecated.
Storagemanager fires an 'store-opened' event which may allow similar functionality to be applied.
Modules that would need to be fixed are mod_auth_anonymous #882 and mod_groups #883