I like the last Cartagena Tom Keil wrote a tut on make realistic sketch with gimp/gmic
http://www.mediafire.com/file/7zcil4tndkhr17k/Howhe used masks and a PS plugin for speed up the creation of mask (luminosity mask to use as layermasks )
I never used masks till now ....(as you i use often instead a ovelayered BW-pencil )but i think is a very good idea.
PS no plugin is strictly needed to create luminosity mask,...quoting Tom
"It is not really difficult to create the luminosity masks without Simpelfilter. Quick and dirty: Use threshold to define your shadows and your highlights, blur the mask images, for shadow mask invert, and paste them in as layer masks. Disadvantage: no transitional tonal range, but in many cases this will be good enough.
More accurate way: Desature the image copies which will give your masks. Apply levels on both, move the blacks up to define your shadows and the whites down to define where the transitional range ends. Adjust Gamma and afterwards adjust contrast to define smoothness of the transitional range. For the shadow image invert and paste them both in as layer masks.I just recommended Simpelfilter because it is quicker and more intuitive to use. "
I use italic for what is for me the best solution ..treeshold is too simplicistic and dirty