The resonance of the plastic or wood would be highly damped by being screwed down to the guitar's top in any case. However, ya wouldn't believe the tiny little details that the best microphone manufacturers attend to when designing their microphones. On a condenser mic, the number of partially drilled holes on the tiny backplates matter. The shape of the interior of the mic matters. The shape of the head basket wire matters. On small diaphragm condensers, it matters whether the end of the mic body near the diaphragm is rounded off or beveled, and the shape of the ports matter.
These are tiny things! A pickup is, for better or worse, a microphonic device. So little things will affect it.
On my old CU22 Soapbar, one of the plastic pickup covers got a tiny bit loose, and caused a really strange warbling resonance when the guitar was strummed until I found and corrected it. This was microphonics at work. I had a Rickenbacker Petty model, and as you might know, Ricks come with a lower pick guard and an upper one. They're made of a lucite-type plastic. The upper one on mine was defective, and Rick wouldn't replace it until I sent the old one back (they are VERY weird to deal with). So I took it off the guitar and sent it back.
The guitar sounded noticeably different simply from taking off this piece of plastic!
I'm not saying that you hear the tone of plastic or wood rings -- I have no idea, and they'd be highly damped by the top, as I said earlier. But it's possible. You're mounting a microphonic device onto a material with different resonant properties with wood vs. plastic. It'd be interesting to see the waveforms in some of the audio software that's out there and have visual/measurement confirmation one way or the other.
