This will be short and sweet. Management Reporter only supports the Dynamics GP Reporting Ledgers feature when using the Data Mart connector.
I was working with a client on some new reports and was just about to recommend using the Reporting Ledgers in GP to record some IFRS adjusting entries. Good thing I did some testing first! Here's a screenshot of Management Reporter 2012 (CU14 I believe) showing the Attribute Filter options. My client uses Legacy, not Data Mart and notice, there is no option to select a Reporting Ledger.
So, for those who want to run reports in Management Reporter against a specific Reporting Ledger, it's possible but Management Reporter must be using the Data Mart to do it! I posted this question on the Microsoft Partner Technical forum to validate the answer and here is the response, to confirm it:
Hope this helps someone out there. I found that there is very little documentation out there about this fact.