as a lightweight deadner? in the sail area or all the hollow channels in the interior?

 

I haven't used it in your examples, but I have used it before. What your proposing is a good idea if you use it in the sail area where the middle speakers go. If your not using that area you can seal it up. A better idea would be to apply a mat over the hole, that way you can remove it later if you want to.
Great Stuff has different products, some expand a lot and some a little so you have to be carefull. The benefit of the expanding foam is that it will apply pressure to the panels and stiffen them up to lower their resonant frequency. Kind of like holding your finger on a vibrating panel except the foam does it all the time. It will also prevent sound from bouncing around in there as well.

I don't think it would have any benefit if used in any other smaller cavities, though.

Overall, I'm not sure if it will be any better than just sticking some mat over the opening. The mat is a lot less messy and more easily reversed, so I would go that route.