This is an absolute gem of a question and I have to say that I've never even considered this problem when I've made very long exposures so I'm sorry that I cannot give you a scientific answer. However, when I have given long exposures in changing light I've always erred on the side of over exposure for if it is on the negative you can print it. Even if you think the highlight is blown out I reckon it will print for film will hold up to 14 stops of contrast. On the other hand under exposure is not recoverable. So my advice is not to worry about the problem if you have information throughout the whole negative.

However, from the replies so far we are all clearly very interested in finding out the correct procedure to deal with this so I will talk to the technical boffins in Ilford UK to see what they say and report back, that is if I can understand the the complicated answer that I suspect I may get.