we had a somewhat similar requirement to retrieve metadata components from any specific managed package in a Salesforce org. Please follow the steps given in the below link. It uses the listMetadata task of Force.com migration tool to retrieve the list of names of all the components of any metadata type from the org followed by filtering out the components name using the managed package namespace and then using Force.com migration tool again to retrieve the filtered out components. Hope it helps.
Retrieve list of components of any metadata type from a managed package