GeeXLab, the successor of GLSL Hacker, comes with the support of Direct3D 12. Its not a full support but most of the basic things are available: command lists (CL), pipeline state objects (PSO), constant buffers (CB) and HLSL shaders.
An introduction to Direct3D programming with GeeXLab is available HERE.
Since GeeXLab is now available with an OpenGL and a Direct3D 12 renderers, here is a quick benchmark that shows the difference of performance between Direct3D 12 and OpenGL 3.2 in a very simple scene: a phong-textured mesh (a torus to be original) is rendered with various polygon density. This test uses one command list, one PSO, one HLSL program, one mesh and one texture.