FreeCAD 0.18 (releasead March 2019) supports such constraints. There are now many "Map Modes" for a sketch besides just "map to face" (the default, and in previ

FreeCAD 0.18 (releasead March 2019) supports such constraints. There are now many "Map Modes" for a sketch besides just "map to face" (the default, and in previous versions the only one). Unfortunately there is not really much intuitive UI available to start that workflow. You have to make the sketch with some/no mapping. And then in the Property editor for "Map Mode" click ... - from that point on there is a pretty nice dialog for chosing the modes etc. In your case one can for example use use the NormalToEdge map mode, specify the degrees of Roll, and then use a Pocket operation on the sketch to make the holes at that angle.