not sure if this is helpful, but if multiple details are on the layout page, you’ll need to enable one of the details to pick something from it. So what about first picking detail, enabling it, then pick the object and once you’ve added your leader just disabling the detail to get back to the page view ?
import rhinoscriptsyntax as rs
# get the detail
detail_id = rs.GetObject("Select Detail", 32768, True, False)
if not detail_id: return
# enable detail
detail_obj = rs.coercerhinoobject(detail_id, True, True)
detail_obj.IsActive = True
# pick object in detail
id = rs.GetObject("Pick object", 0, False, False)
if not id: return
# TODO / read user data or do something with picked object
# disable detail
detail_obj.IsActive = False
Or do you want to pick object inside the detail without having it activated ?