Did some digging and found it finally after the 20th search term combination. It's /LibraryReadOnly and it is run off of the "Media Center 12.exe" not "mc12.exe" just to clear that up.
Regarding that, if you can't do the /? for some reason, it would be helpful to have the startup switches documented in the Wiki or help file or somewhere like that.
Back on topic for this thread, I may have found a partial solution. If you do a MCC_LOAD_LIBRARY indexed to the main library on the read only client, it will reload / refresh any changes made by the read/write client. The only hitch is I can't see a way to use that command and specify read only. If you could, you would be downtown and feeliin' good. Unfortunately how it works now is if you happen to close the read / write client, and envoke this command, it will open the library read write on the headless server. Then things get complicated to get back in step. You have to close the MC running on the head less client and reopen read only manually.
Do you know a way to invoke the MCC_LOAD_LIBRARY with a read only switch? Perhaps you could add a separate MCC_LOAD_LIBRARYREADONLY ? I think that would solve a lot of peoples' problems while enabling us to run a headless media server and mange the library from a desktop, then simply refresh the library on a schedule or on demand. I would be satisfied with that approach, and there are more than a few of us wanting to implement this type of architectural solution.