There is currently no way to filter resulting objects when fetching related_objects.

The attached patch adds support for this feature, using the following syntax:

{def $res1=fetch( content, related_objects, hash(
    'object_id', 2413,
    'all_relations', 'common',
    'related_class_identifiers', array( 'image' ),
))}

For consistency sake, it could make sense to change the patch a bit to support the same syntax as fetch/content/list: class_filter_type:(include|exclude), class_filter_array:(class1,class2,classN). The underlying SQL perfectly fits this evolution.