Helmut Kudrnovsky wrote
r.in.xyz
r.surf.idw
v.surf.bspline
v.surf.rst
None of these, or any of GRASS's other surface interpolation tools that I'm aware of, consider breaklines. Although a feature request was filed 4 years ago:
http://trac.osgeo.org/grass/ticket/793You could probably generate hydro flattened surfaces separately and combine them with the DEM manually using the raster calculator, but unfortunately I don't think there is a one click button.