block_piece_select.py

import rhinoscriptsyntax as rs
import Rhino

def ModifyInstancePiece():
    rs.UnselectAllObjects()
    go = Rhino.Input.Custom.GetObject()
    go.SubObjectSelect = True
    go.Get()
    if go.CommandResult() != Rhino.Commands.Result.Success:
        return
    objref = go.Object(0)
    part = objref.InstanceDefinitionPart()
    if part:
        val = rs.GetUserText(part, "counter")
        c = 1
        if val:
            print val
            c = int(val) + 1
        else:
            print "no user text yet"
        rs.SetUserText(part, "counter", c)
    rs.UnselectAllObjects()


#hold down ctrl+shift to sub object select
ModifyInstancePiece()