You can implement fog in the final combiner. The fog register’s RGB contains the color of the fog in both the general combiners and in the final combiner, and in the final combiner, the alpha of the fog register becomes available. It contains the fog factor.
(1-A)B + AC just happens to be the fog equation. The + D, well, that’s just another adder.
The fog factor is computed just like it is normally. You can set LINEAR, EXP, or EXP2 fog, and you can use a fog coordinate or one of the three fog distance modes (eye plane, eye plane absolute, or radial).
In fact, fog is more flexible with register combiners, because instead of being forced to blend using the fog factor, you can do other things with it. For example, you could output it as the G register (your pixel’s alpha) and use it in an alpha test to clip away pixels that were more than or less than some given radial distance from the eye.