Here is some reading materials for you:
http://www.michigan.gov/msp/0,4643,7-123-1586_27094-10953--,00.htmlFrom the FAQ section:
2. I have a replica of an antique pistol. Is a License to Purchase or Pistol Sales Record required in Michigan? Is it necessary that I obtain a Michigan Concealed Pistols License?
MCL 28.422 No, antique pistols made before 1898 and replicas of antiques that use black powder, matchlock, flintlock, percussion cap or similar type of ignition system do not require a License to Purchase.
The pistol is still subject to all concealed pistol licensing laws. From this it would seem that you can OC a BP pistol, however you should consult a lawyer before doing so, if you have any doubts. I know there are several member who carry them. Member CrossPistols comes to mind. To legally OPEN carry a modern pistol [Non CPL holders] it must be carried only by the registered owner in a proper, fully visible holster (via a Purchase Permit ).
Hope this helps.