for raised detail I have used fisket film before. cut a piece the size of the plaque, stick it on, and using a new blade, carefully cut around the raised lettering. Depending on how you plan to paint you can remove the letters, or the surrounding, leaving the mask in place.
Another idea is to paint the plaque, then use a firm sponge to apply paint to the raised lettering surface only (tray with paper towel might work too).
Personally I like the idea of doing a bare metal foiling, the cutting would be the same as if you were masking with fisket.
As far as acrylics, it is certainly OK to layer. the rule of thumb I hear a lot is that you can lay additional coats within 3 hours of each other, or after waiting 24 hours. I dunno on your hairspray idea (I have seen this used to help in weathering, but not the way you are describing). There are also liquid masks that you can use to paint them on. Usually they are a latex base. I have used one called MicroMask, but it doesn't seem to react well to my can-spray primer, I have a lot of trouble removing it. There is another one, a white bush on frisket, think it's made by a company called Graphix or something similar to that. I hear good things about it, and plan to test it out...