Paul, you missed part of the picture. Suppose we have a different kernel, which does not have the same bug as Linux has,and that, further, depends on that register being visible? We can't know that such OSes exist, but we do not know that they do not. We'd have to at the very least test some of them. We've always tried to avoid being Linux-centric in coreboot and for the most part have succeeded. Further, hidden registers create their own problems.
This problem has no clear solution. I've always felt that in all cases, we should err on the side of opening up the hardware, and not hiding registers.
ron