There’s been a thread over on the darktable-user list for some days now about how to copy masks. Terry Pinfold posted this yesterday:
"Found this solution online to copying masks between modules. Works fine in 2.6.2.
"When you choose drawn mask there is a drop down menu labeled “no mask used” to the left of the edit and polarity buttons. When you click on it all of the drawn masks used in that image will be listed. I think you’ll even get groups. If you’ve got a drawn mask combines with a gradient in one module you can use the same combination in another. Instead of being listed as path 1 and gradient 1 they will be listed as, say, exposure group. "
I think he means you can select from the drop-down list in a different module. Haven’t tried it myself yet. Hope it helps!