Insteon also has a workaround.
They have kits with an Insteon fixture mounted module and a Insteon SwitchLinc. With it you again wire the line and neutral to the existing wires in the switch box to power the SwitchLinc and then it controls the fixture module. I tried one and in my older homes very small fixture boxes. The module didn't fit.
UPB you can do the same thing (its not a kit but they have the same fucntionality with a switch and a hardwire relay or dimmer module). Luckily for me it did fit in a standard round ceiling fixture box. Not sure if it would fit in a Ceiling Fan rated box.