This OpenSCAD module will calculate an inner bevel for an arbitrary part. You need to specify the height of the z-plane at witch to create the bevel. Replace the module part() with your own part. This is based on an ingenious idea from GregFrost (http://www.thingiverse.com/thing:135408). He used it to create an (outer) bevel on a arbitrary shape. The idea is to minkowski() a small square with the shape creating a slightly larger shape and subtracting the initial shape, creating a thin outline of the initial shape. In a second step the from of the bevel is minkowski() with the outline. Unfortionally there are two limitations to this: Enjoy.