I (cyrfer) wrote a GLSL shader to examine each channel of image data from the texture. For the case I'm trying to use (PF_L8 --> PF_A8), samples for each color channel is zero and the alpha is always one (WRONG). If I let the texture be luminance, then samples for all color channels have the expect image data. I'm pretty sure this implies that a bug occurred when writing the image data to the GL texture object.
Original Mantis Ticket: http://www.ogre3d.org/mantis/view.php?id=330