I agree with most of what you said about Chicago.  In fact, I've always said that Chicago was more conservative than a lot of smaller cities in the Great Lakes region (namely Cleveland, Detroit and Milwaukee) if for no other reason than a larger percentage of people who fit into one or more of the categories you named (person of color, union worker, "lakefront liberal," and I would add bohemian/counter-culturist to the list as well).  

However, I don't think that's enough to say it couldn't count as a "third coast." New York has a republican mayor and California has a republican governor, so it's pretty obvious that the "first" and "second" coasts have their conservative segments as well.  As far as Chicago culture being bland compared to the East and West coasts, you're probably right about that too.  But it's not as if people on the east or west coast don't have their criticism about the other coasts' culture as well.  East Coast people tend to think West Coast culture is too fluffy and superficial and West Coast people think the East Coast is too uptight and high-strung.  So I don't think how Chicago/Great Lakes culture is viewed by people on the coasts shoould really have any bearing on whether or not it's a "coast."

Besides, I don't think the purpose is to compare the Great Lakes to the Eastern or Western seaboards.  I think its purpose is more to differentiate the major urban centers of the Great Lakes from the Midwest at large (which is seen as mostly small-town and conservative).  And I don't think you can disagree that when compared to the rest of their states that places like Chicago, Detroit and Milwaukee do seem like "ultra-urban" and "ultra-liberal" oasises.