step-down transformer

YES, I HAVE USED IT for the Japanese imported Luxman. AJD-1400 I think. 

Theoretically, yes a backwards step-up xformer should do the trick. I've never tried it.

With an extra component and an extra pair of interconnects, you might as well just use a switch and have both connected to the MM input. Something like a Rek-O-Kut two way switch would be fine.

You could in theory do that, but be mindful of the impedance transformation.  If you used a 1:10 SUT, then you want your MC inputs to have ideally 470 ohms impedance, so that with the SUT hooked up backwards, the MM cartridge sees 47K ohms. I don't know anyone who ever tried it.