Hey Erin,
The easiest way would be to create a Marketing Suspend field in SFDC and utilize support to sync the two fields. As of a couple of weeks ago, their tool that they use to manage that was not functioning (I had a similar request). They suggested creating the field in SFDC, let that field sync to Mkto, then have a trigger campaign that when the Mkto Marketing Suspend field is checked, check the field that syncs to SFDC. And vice versa, if they are unchechecked in Mkto, have a campaign to uncheck the field that syncs to SFDC.
Interesting Moments are a great place as well, but it depends on how often your reps check it. Also, if the Lead is active, the list could be pretty long and tough to navigate.