I travel to the Detroit area twice a year and have futilely searched for Bike rentals. I even contacted the apparently very active Ann Arbor Bicycle Touring Society to no avail. I also tried the famous Wolverine Bike Shop in Madison Heights. Other than hauling my own bike, I borrow my brother's heavy comfort hybrid; good enough for me to get some exercise. Another suggestion, I have thought of is to buy a used bike out there; I have seen some acceptable road bikes. You might be able to sell them afterwards
Most of my riding in Detroit is in the Northeast suburbs and I have found on most major roads I have to accomodate to riding on sidewalks for my cycling sanity since many roads lack shoulders, so my brother's bike is fine.
I'll keep a lookout on the Great Lakes Forum for replies to your query. I would be very interested to know about them. If you get a chance, I would also like to hear of your cycling experiences out there, and if you have any questions I might answer, please feel free to contact me. Though I have no specific answer to your question, I'm glad I was reading the Northeast section forum, and I certainly would have missed it if you only posted to the Great Lakes.