I do know that using the db_* utilities are only applicable to the BDB backend. As far as I know, it's the most mature of the backends to use with 2.3.43. (If I'm wrong in that, do let me know.)
2.3 is not a supported release series. I would strongly advise upgrading to a supported release. But yes, back-hdb/bdb are the two mature backends for use in the 2.3 series. I would only ever consider a "safe" backup of bdb itself to be when slapd is shut down, and after db_recover has been run. Then you can safely back up the *.bdb and log.* files. Puting slapd in read-only mode is not necessarily sufficient, as you need to force a BDB checkpoint prior to backing up the BDB db.
--Quanah -- Quanah Gibson-Mount Sr. Member of Technical Staff Zimbra, Inc A Division of VMware, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration