Ok if you are going to do it then maybe it's time to think outside the box.
Instead of magnetic speakers you could use piezo disks, these are light and you could use more than one to increase the sound producing area. You are after a high freq noise anyway.
Then instead of thinking about an "amp", why not generate the pink noise directly with a high power digital circuit? Battery weight is at a premium, so you could make a push-pull noise generator using FETs, it doesn't need to have any analog amplification as such, just to run full bore digitally with a basic digital noise source (counter IC?) controlling it.