You can indeed use the WIC framework from C++ to access image data and your software will be able to take advantage of any installed WIC codec automatically: adding raw support to a WIC-enabled application is simply a matter of installing the appropriate codec on the end-user machine. This is in fact exactly how the FastPictureViewer Codec Pack extends Windows Explorer, Windows 7's Media Center etc.
All our raw codecs currently returns 24bpp RGB data (specifically: WICPixelFormat24bppBGR, but you must always query for that using the appropriate mechanisms as this may change in the future).
Most of the time in normal use - keep in mind that these codecs are primarily Windows Explorer extensions used to build thumbnails and fast previews - the images are created from the embedded previews, which are 24bpp JPEGs. When the RGB data is made by converting the raw information, the output is reduced to 24 bit RGB so the same pixel format is returned for the thumbnails, preview and full-size images. This format, incidentally, matches the hardware pixel format of most display cards...
Some of our raster codecs (e.g. PSD, HDR, EXR, DDS, TGA, PNM) can return higher bit depth, up to the maximum supported by the underlying format.