Hopefully, 'real soon now', we will be able to do something to help you.
In the meantime, keep them as separate packages. You're trying to get your mission 'out there' not the addon, which would put people off downloading your mission at such a large size (a perception thing)
we look forward to testing this 2 year old infant of yours. When the time comes, just post two separate urls in your topic in the Beta Forum.