A quick hack consists in removing the test for SLAP_GLUE_SUBORDINATE() in monitor_subsys_database_init(). This will expose subordinate databases and thus should allow back-bdb and back-hdb to hook their cache stats stuff. Probably this is the best approach. I would complement it by adding a olmIsSubordinate attribute with TRUE value, and/or a olmSuperiorSuffix containing the suffix of the superior.
An even better approach would be to append subordinate databases below the superior one... but I'd leave this for 2.5 :)
p.