I think the brush tool that would work best is the one you find in the effects stack.
As you apply effects in Snapseed, they get added in layers. At the top right of the screen, next to Save, is a number that shows the number of effects layers you've applied. If you tap on that number, you get a pop up in the lower right that shows the stack of effects layers. If you tap on one of those layers, you get a fly-out menu with a brush icon. Tap that, and you can apply that layer's effect to any part of the image with the brush.
So if you want to brighten the face by raising the Brightness setting, for example, you apply the Brightness to the whole image until the face looks right (even though other parts of the image will now be too bright). Once that's applied, you then go to the layer stack, pick the brush for the Brightness effect, and brush it on the face just where you want it. (When you select the brush icon, the effect gets masked off the entire image, and your brush strokes bring it back.)
It's not intuitive, but once you get the hang of it, it works very well.
As you apply effects in Snapseed, they get added in layers. At the top right of the screen, next to Save, is a number that shows the number of effects layers you've applied. If you tap on that number, you get a pop up in the lower right that shows the stack of effects layers. If you tap on one of those layers, you get a fly-out menu with a brush icon. Tap that, and you can apply that layer's effect to any part of the image with the brush.
So if you want to brighten the face by raising the Brightness setting, for example, you apply the Brightness to the whole image until the face looks right (even though other parts of the image will now be too bright). Once that's applied, you then go to the layer stack, pick the brush for the Brightness effect, and brush it on the face just where you want it. (When you select the brush icon, the effect gets masked off the entire image, and your brush strokes bring it back.)
It's not intuitive, but once you get the hang of it, it works very well.