Hello Ryan,
Thanks for your post, sorry to say, we don’t support this event in FeatureSourceMarkerOverlay and I don’t heard we have the plan recently.
You can vote this enhancement to helpdesk.thinkgeo.com/EnhancementTracker. This captures the enhancement request and provides visibility to the customer letting them know that the enhancement is on a list somewhere and that popularity of the enhancement helps set the priority of when the enhancement would be added to the product. This option doesn’t carry any cost for you.
Or if you need this function immediately, your account rep can contact you for a professional services.
Regards,
Gary