At FEM Engineering a quad mesher was coded, and used with modelers such as AutoCAD, which do not come equipped with one. This mesher was used successfully in various projects. Source code is made available here, under the name Cfmesh.
CQMesh is another quad mesher for which source code is available. Its method of converting triangles to quads might be worth implementing in a core-and-fill mesher; the method in Cfmesh is rather ad-hoc.