Just a thought, but have you tried using an add-on image loader like Brucey's Freeimage.mod module? It supports many formats:
GIF, BMP, DDS, EXR, HDR, ICO, IFF, JNG, JPG, JPEG 2000, KOALA, LBM, Kodak, MNG, PCS, PBM, PGM, PNG, PPM, Photoshop (PSD), Fax G3, SGI, RAS, TARGA, TIFF, WBMP, XBM and XPM.
Several of those (GIF, IFF, LBM) are always indexed color, so at least in principle it's possible that it also does index BMP's...
GIF, BMP, DDS, EXR, HDR, ICO, IFF, JNG, JPG, JPEG 2000, KOALA, LBM, Kodak, MNG, PCS, PBM, PGM, PNG, PPM, Photoshop (PSD), Fax G3, SGI, RAS, TARGA, TIFF, WBMP, XBM and XPM.
Several of those (GIF, IFF, LBM) are always indexed color, so at least in principle it's possible that it also does index BMP's...