thanks both, @Tom_P this led me to RhinoObject. GetSelectedSubObjects() method, which seems to retrieve “ComponentIndex”, which I hope can be used to modify object attributes in multiple block instances at once.
I still think that a visual feedback using existing Sel* commands would be useful, as sometimes you want to check where sub-objects are, visually filter some of them out and then apply changes.
import rhinoscriptsyntax as rs
b = rs.coercerhinoobject("ff2d1ab3-814f-41d7-98da-d29ef358d9ad")
c = b.GetSelectedSubObjects()
