Hey man, here are a few thoughts on the gouache technique you were talking about...
first of all, i know thomas blackshear and david grove both have used a gouache rubout technique. google search both of them, they are both incredible illustrators and fine artists. (blackshear is quite a bit religious though)
sample by thomas blackshear
samples of gouache rubout by david grove.
and over on mike butkus' thread there's an example with a morgan freeman looking head.
but, these are all more like "rubout" techniques than underpainting/grisailles.
from what you were saying, i think it will work provided you keep the paint layers very, very thin. if there's anything close to impasto, it probably will be a mess. if you want to use acrylics over it, use matte medium with NO water. repeat NO water. use water to clean your brushes, but dry them out before painting.
instead of water you can save a ton of money and buy latex paint thinner from the hardware store. much, much cheaper.
anyway, when you paint the acrylic over the sealed gouache, just thin your paint with the matte medium and make it translucent. from there it should all be cake. if you do it in multiple layers of acrylic, you can get very smooth rendering.
anyway...regarding the black and white gouache one... it looks right, but you also need to have everything that will be in the light up in value to a middle gray. from there, pop the lights pretty hard. in your head you should almost think that you're doing a "chrome" version of the subject. you can always kill some of the contrast later by glazing/washing over it. that's where judgment comes in--what to keep and what to lose from the chrome version. just paint over it in layers and it should be fine.
note: i don't know what painting with a water-based paint like gouache over spray fixative will look like... not sure if it beads up a lot or if it still stays smooth. experiment and find out!