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Right now you can find it in the module dev forge:
http://dev.xoops.org/modules/xfmod/project/showfiles.php?group_id=1359&release_id=1224#selected
I have seen this module working, so I know it's possible, but as you will notice, it is difficult. I suggest installing it on a fresh installation of XOOPS and then sharing the tables from your main site and all of the other sites through that one. Nekro is helping me get mine working. Once it's all running the right way I will be able to see what needs to be done to make it work on other sites. I am also planning on throwing Nekro some cash as soon as I can for all the help.
Magick can never be restrained, but when freely given is thrice regained!