It should be fairly easy to include Policies and Policy Templates. ACL entries will be a challenge, though.
I haven't really looked at Users and User Groups yet, but it should be doable.
Transporting installed packages is also on the roadmap, but a ways off.
The down side right now for what you want is that the exportObjects method exports on the basis of category and will only accept one category -- the thinking being that all the objects for your extra will be in the same category (or have its namespace).
I may refactor it someday to provide the option to get everything from the config file, regardless of category, but the config file would get pretty involved and it won't happen any time soon.
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MODX info for everyone:
http://bobsguides.com/modx.html
