We are in need of using 8 bits gray scale texture, cause they give a good looking grey scale textures and save a lot of video memory.
So how can i add support to this texture format ? in openGL this texture format is called GL_LUMINESCENCE and in direct3D D3DFMT_L8.
You have to add the new image formats, a loading routine, and finally the correct texture handling. Also be sure that you use Irrlicht SVN as OpenGL did use ARGB8 only in previous versions (i.e. always 32bit).
Try to follow the compressed textures patches. There all necessary changes for DXT_* formats are included. You'll probably have to do the same things for your format. You can find them on my patch page, also note that there are dependencies you have to take care of.