Post #909: 
roaoao:
Try this code in Custom tab:
row = L.row(True); C.active_operator and hasattr(C.active_operator, "threshold") and row.prop(C.active_operator, "threshold")
Thanks! It works okay. You need to check the box “unselected” in the “t” oanel and then unselect it again in order for the value change to take place, so something is a bit wierd. But it’s getting there!