There are materials of the Trans-elect website that details the CMS purchase,  http://www.trans-elect.com/. Under a Q&A section is the following:

Q: Will Trans-Elect put the METC transmission system in an RTO?
A: Yes, in accordance with FERC requirements that all systems must join an RTO.
Q: Which RTO will Trans-Elect choose?
A: Either the Midwest Independent System Organization (MISO) or the Alliance will be selected. Trans-Elect has 40 days from the announcement to make a selection. Trans-Elect is assessing the two organizations to make a determination as to which one will best serve the interests of Michigan consumers.

Looks like there was substance to the Consumers energy rumour.  Consumers are currently part of Alliance, the fact that the new owner wanted to make a decision on RTOs suggests that they are moving to MISO (they would have looked at the ARTO structure as part of due diligence).  Will let you know when the decision is made.

regards
Mike