glDrawPixels and register combiners

We’ve been trying to render a YCrCb video frame in memory onto our GeForce3 by using DrawPixels and setting our register combiners to perform the color conversion. The results seem to apply the register combiners to either primary colors or textures and we were hoping to avoid rendering to a texture as the pbuffer on the card is our only rendering context. Is there another way to pipeline the results of our drawpixels call so the register combiners are applied to the frame?

hm… glTexSubImage and then drawing a quad is no solution? (drawing in pieces of 256x256 for example… you’ll have to set up a wrapper function myglDrawRectangle(…))

We are currently using glTexSubImage2D, but speed is a real issue for us. We were hoping to be able to use the results from glDrawPixels in the register combiners and avoid the overhead of copying/rendering to a texture.