http://trac.webkit.org/changeset/62035 introduced a new method of drawing border-radius using paths. Right now, this new code is only enabled for some platforms. To enable the new and much improved code for Wx, GraphicsContext::clipConvexPolygon() needs to be implemented, and then Wx should be added to the list of platforms that set #define HAVE_PATH_BASED_BORDER_RADIUS_DRAWING in RenderObject.h I would like to note that Wx already has a function implemented called GraphicsContext::drawConvexPolygon(). So hopefully it is straightforward to use some of that same logic for clipping instead of drawing.
I don't think we have wx port now. I am marking this as "RESOLVED WONTFIX" and adding few Webkit Engineers to confirm that it is right approach. Thanks!