Hi Will,
The process of adding a profile of ISO (or even a new metadata schema) is going to change in 2.7 shortly as mentioned in this proposal:
http://trac.osgeo.org/geonetwork/wiki/pluginprofiles
There is a basic guide to adding a profile/schema with the proposal (which needs some improvement and to be added to the documentation) and a link to a 2.7 based test installer that you could use.
You could still do this for 2.4.x but the process has more steps and may require java code modification to recognise your schema. I hope to commit the plugin profile/schema code to 2.7 trunk this week/early next week - would have been sooner but I’ve been held up by some changes to support proxy credentials on the resolver used in this code.
Cheers,
Simon