I have the diff out on my ITA Miata, and I was trying to find if the two diff bushings that hold the assembly to the car are legal to replace with poly units. It seems that all suspension bushings are open, but I wouldn't count the diff as part of the suspension.

9.1.3.5.d.6
Bushing material, including that used to mount a suspension subframe to the chassis, is unrestricted. This includes the use of spherical bearings, so long as no suspension component is modified to facilitate their installation. Retention of spherical bearings by use of tack welds is allowed, as long as the welds serve no other purpose.

Maybe the question is: is the diff part of the suspension subframe?