I agree, if you must, declare an emergency, but you would need to have a good reason for not going to an alternate (such as the weather conditions were no better).
The case you mention is interesting. I would have guessed with a good barrister the prosecution would not have been successful.
However, the law makes it illegal to even start the approach if the RVR is inadequate. Clearly this pilot not only started, but concluded, the approach knowing that. Perhaps if he had asked, and AT had confirmed that the RVR was well above minima in the first half of the runway, and he had declared he would commence his approach on this basis with the intention of landing only if he could stop within the first half, he would have escaped prosecution.
I have been in almost that exact situation. The airport (or should I say grass strip I was going to) was exactly on the edge of the Ha when I arrived. One side of the line of sea fog it was CAVOK, 3,000m the other side it was probably 00. As it turned out I was lucky. I hung around for about 20 minutes as I had plenty of fuel and landed when the strip was clear. As I landed the Ha was settled on the forest line at the far end which could barely be seen. Of course an hour later it had all burnt off. The go around, had it been required, would have been on instruments. I have to admit an engine failure on the go around would have been interesting but as you can imagine I had very little intention of going around
. I also recall another occasion in Alderney arriving back as the bank of fog started to roll in form the sea. The runway was just starting to go into the fog as I started the departure roll (but he RVR was above minima), climbing out through the fog bank on insturments into crystal blue skys and CAVOK both above and below across the channel. I was the last flight out that day, and the next.