Your conclusion is right, but the understanding of getting there is a little off. A material that does not conduct vibration is a damper. This loses sustain. We all agree that using foam saddles, which have an extremely low conductance, is bad, yes? Its not the fact that they don't conduct vibrations that causes higher sustain. Its the fact that they are hard enough to not absorb the vibes, but its a slippery material that causes less of the energy to be conducted into the material. Each wave the string makes causes a tiny amount of energy to be absorbed in making the saddle move. If the saddle is slippery, like tusq or teflon, then the string keeps more of its energy and is able to vibrate a little longer.
The golden point, is that you want something that doesn't conduct OR dampen. Something in between. This causes the string to hang on to more of its energy, and it makes the air the actual energy sink. Energy will go wherever there is less energy. Like a sponge, if you have a dry sponge it will absorb more water than a wet sponge. If the saddle conducts energy easily, then it will be passing the energy along to the body faster than the air around the strings, causing more energy to go into the body than the air. Conversely, if you have a saddle made of something with a high damping, this means it is able to hold more energy, so it will suck up more than the air.
Of course, there is the fact that you do WANT the body to take some of the vibration, to get the most out of your tonewood. I know you say and think the pickups are what makes the sound and the wood makes no difference, and you are part right. The pups are what take in the signal, and they are NOT effected by the wood vibrations the same way as an acoustic makes its sound. The only sound picked up is coming from the string. BUT, when a vibrating string is attached to a hunk of wood, it causes the wood to vibrate. In turn, it vibrates the string as well, in a big loop. They reach a sympathetic resonance, where they both vibrate at the same frequencies. Some wood types dampen certain frequencies, effectively sucking those frequences from the vibration of the string. The pickups only care about what the strings are doing, and they make the signal from that, but the strings are affected by the wood vibrations. So, the wood tone is in a way being picked up by the pups, by proxy. This is why the best sounding guitars have very resonant woods. Using different types of pickups form a second frequency filter as well, they respond to certain frequencies the strings are making. But if the wood has already removed those frequecies from a string before the pup gets it, then it doesn't matter if the pup is stronger in certain frequencies, the string isn't providing the freq for it to make signal from.
Example, the JB is a trebly pup. Rosewood is a dark wood, in the sense that the oil in it dampens high freq, in contrast to maple, which has a lot of highs, but doesn't conduct lows as well. The very same pup on two bodies will sound very different, a solid rosewood guitar body is going to have less highs available for the pup to pick up, as opposed to the solid maple body. The rosewood will sound dark, the maple will be bright. Of course in the real world we are using a combo of woods in differing thickness, solid rosewood or maple bodies are very seldom used because they are so heavy. Usually its a thin peice on top of a lighter wood, so its sonic properties play less of a role.
Ok, I need to get back to work. That was a longer post than I expected.