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I'd say it going to take a lot more than a hack to be able to adequately handle a job of this size. I don't think there are any modules/hacks available at present that have the ability to handle such a large job. On one of the sites I help run we are using a list server for the 4500 or so users signed up for newsletters.
Myself and a few others have been throwing ideas around of paying a developer to write a module for XOOPS specifically to be able to mail out to very large user bases without killing the server. This is not a trivial task as the code must be very robust, efficient, scalable and secure. If you are interested let me know.