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When loading a Session add the Session patchfiles directory to the MidiPatchManager search path and only process/parse the files for that directory rather than refreshing/reparsing all the files. Similarly for unload, just unload the devices that are from the Session specific midnam files instead of removing the path and refreshing/reparsing all the files. This will not remove the "system" midnam files as they are always added first and duplicates from the session patchfiles directory are ignored.
Please see the Ardour web site at http://ardour.org/ for all documentation..
For information on building ardour:
http://ardour.org/development.html