I came up with an idea how to make a node setup to render wire frame in Cycles with adjustable thickness.
== UPDATE ==
I added another method based on the parametric (barycentric) coordinates of the triangle using the Geometry node output.
Unlike the UV method it DOES NOT need the UV map, but it will shade the edge of each triangle so if you subdivide the mesh a lot the wires will be tiny.
So you have now two node setups for quads or triangles.