By merging the open archive plugin with the openfire monitoring plugin we've created a new monitoring plugin which has chat and group chat archiving (from monitoring) as well as XEP-0136 Message Archiving support (from open archive).
The base plugin is the Openfire Monitoring plugin.
Only issue is that the archive plugin is GPL2 licensed and the monitoring plugin is Apache licensed (right?). Best I can tell the two licenses combine fine:
http://www.gnu.org/licenses/gpl-faq.html#AllCompatibility
We use the plugin internally but I'm happy to share the source (took some work integrating the two) if anyone wants it.
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EDIT
Work and a new baby got in the way of my best intentions, but despite being a little late we're happy to let everyone know that the updated Monitoring plugin is in Openfire trunk.
We wanted to originally add the plugin as a separate Maven project since the Openfire Ant build is a touch archaic (but functional! 
) However due to the way the Maven-Openfire Plugin works and an issue in DWR 1.1.3 (it does not resolve classes outside its own lib folder) we had to put the new Monitoring plugin src in the Openfire trunk and compile it the Ant way.
The upshot of this is that all the graphing and statistics pages work just fine.
A change to the database schema of the Monitoring plugin was made to support the XEP-0136 spec. We've tried our very best to handle the database upgrade for all the supported DBs as best as possible.
Source:
http://svn.igniterealtime.org/svn/repos/openfire/trunk/src/plugins/monitoring/
Latest build (1.3.0) of the plugin is attached.
Any bugs please contact me or ideally file in JIRA (I believe you can file them against leonroy).