Can the A6 engine use filters to modify colors and add motion blur etc.?

This is speculation, but it should be possible.

If you have Pro, you can probably use the render to texture, along with a shader to get some effects for the screen. Another option would be to use the SDK and directx to get and lock the framebuffer, apply your filter and release it. Im not sure exactly, when this should be called, but as a render_entities function sounds like the right place too me. I guess it would depend if you want the filter to effect view entities and panels on screen or not.

Another think to keep in mind is that a filter for the wholescreen each frame can be very cpu consuming.

Shaders are where this comes into play and render to texture.

The screenshots have been taken down, but this had something to do with it:

http://www.conitecserver.com/ubbthreads/showflat.php?Cat=&Number=349748&page=0&view=collapsed&sb=5&o=21&fpart=1