layers or Lasso
Actually, Aperture has them, but restricted to adjustments. If you think about it, how brushed adjustments are working, they are implemented as layered masks of brush strokes. Only there two severe restrictions built deep into the design of Aperture - it is restricted to RGB-images - no alpha channels and transparency, and no compositing - you simply cannot combine parts from different images. If this ever wll come to aperture, then in a new release, not in an update to the current version.