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.

column attribute filter
A screenshot of the Attribute Filter in Mgmt Reporter, and Reporting Ledgers are not an option I can select.

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:

MR reporting ledgers
Confirmation that a data mart is needed for this.

Hope this helps someone out there. I found that there is very little documentation out there about this fact.