Has anyone tried resistor loading the primary of a MC SUT ?

Loading on the primary side would have the effect of reducing the net output resistance of the cartridge, theoretically making it more capable of driving a low impedance on the secondary side.   So, if you have an LOMC with a high-ish internal resistance but low signal voltage output (so it needs a relatively high turns ratio), like the Denon DL103, then it might make some sense to load on the primary side.  I don't know how or whether that might color the sound, apart from the impedance matching effect. I think there is a good white paper on loading the SUT to be found on the Jensen transformer website.

There is little point to loading the input side.

The reason is that most of the loading issues that you will hear are caused by the transformer itself and not what is driving it. The loading will be different depending on the cartridge used, but the point here with the loading is to prevent the transformer from ringing.

The cartridge won't ring at audio frequencies so there's no point in loading it. It does ring in ultrasonic or radio frequencies, but the transformer is likely blocking that so loading the output of the transformer is really the effective thing to do.

If the loading is too light (value too high) the transformer will ring, if too heavy (loading value too low) the transformer will roll off the highs. This is why the correct value is known as 'critical damping'.

Ralph, whether it is only the phonostage that needs loading or also the cartridge is debatable. The general terminology is "cartridge loading". So adding resistors to primary should load and damp the cartridge.