I think I can figure out how to do this, however I can't say that it wouldn't require core file modifications (because it will require them)...
Basically you need to find in your files where it's looking for table_prefix.'user as the table. once you find that get rid of table_prefix.' and replace it with whatever text you want for your shared table of users.
If someone is serious about doing this I wouldn't mind helping you out.
-John
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