No, it's come up before. There's actually been clarifications that even having metal sleeving/etc around an otherwise stock material bushing is no go, even if there is no perceived stiffness benefit. It does indeed mean that you must keep the same materials, including the same hardness of otherwise same materials (rubbers), and can't change proportions of bushings that have metal sleeves/etc to have more or less. You also cannot poly-fill bushings that have gaps, which was another common loophole "But I kept the stock bushing! I just filled all the empty space with poly.."
Sandwiching things or adding additional parts equates to changing mounting points/geometry, which is also prohibited.
I haven't read the rules in some years, but there is an overall clause in them or always was before that says something to the effect of "All other changes than explicitly listed are prohibited."
The rule wording is too ambiguous if that's the case. It should just state something to the effect that unmodified, stock suspension bushings are required.