I have overridden uvm_objects by instanceby using the path of the component instantiating the object and a wildcard as a path (not sure if the wildcard is really required).
Something like if I remember correctly:
set_inst_override_by_type("path.to.agentA.*",
read_q_manger::get_type(), my_read_q_manager::get_type());
This should override any read_q_managers instantianted under agent A with my_read_q_managers.