Because of some very local details I went for a drawn mask.
First a rough contour and then local fine tuning including changes of feather radius. However, trying to edit in restricted edit mode, adding and deleting nodes, (cnrtl-)clicking and moving individual (feather) nodes was very, very cumbersome. Sometimes it worked, most of the times nothing happened. What was supposed to be a quick local edit became an hour at least (and lots of crtl z’s).
Am I missing something in the user interface?
This was using 3.4.1 on Ubuntu 18.04.