I recently discovered a mirrorless camera feature that I hadn't known about before.
When shooting for B&W images, put your camera in RAW format mode then change your image setting, Record Mode > Film Settings, to B&W Standard or B&W Dynamic. This will change your viewfinder to B&W and you will see your subject as a B&W image. When you take the picture, you will see a B&W image in your image review. However, this is still a RAW image. When you import the RAW image, it will probably revert back to color (this depends on your software and the settings you have) or, if it stays B&W, you can do a re-set and it will then return to native color. Once it is in color, you can use all the tools of your software application, which are far more powerful than what the camera can do, to manipulate the image to a B&W image. Having the viewfinder in B&W, though, is a huge help in "seeing" what your subject is going to look like as a B&W image.
On a GH2 this change can be made in the Quick Menu settings in about two seconds, or you can add the menu item to a custom function button. I would assume that other mirrorless cameras can do the same trick with similar settings. Try it out, it's a big help for composition.