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I'm not sure if this is currently possible by utilising work arounds but Tableau 10 soon to be released has built in functionality for Custom Territories/Boundaries
To see some of the new features follow the link below
Tom W share this a previous post. I'm not sure if this is what you are going for, but it provides a boundary by zip code group, and still allows the shading of the individual zip codes by a measure.
To create, make a dual axis map with the second layer as a line instead of a polygon. Color the line by group.
Thematic polygon maps_TW.twbx 66.2 KB