Torben, my son John, who has become a geek in the PHP/forums realm, has seen what you are doing here. He could not help coming with his own solution. I know that it is a bit late to contribute, as it is obvious that you had a hard time to get the job done this far
, but I must admit that what he as in mind looks great too!
Yesterday evening, he wrote a PHP sniffer-script that is currently filtering all the FWSF pages hierarchy along with the attached images and files. Theses pages anchors get altered in such a way that their reference is remapped to each new target location and the links related to the dead forum, which have to become inoperative, get grayed out to make them look dead indeed. Mostly all other links will hit their target. And here comes the most handy part: seamless forum-like navigation, as long as all index pages get properly restored along the way.
The result should hopefully provide a simple huge bunch of files that would allow us to navigate the original hierarchy. Only some accessory navigation anchors would be grayed out along with those dealing with member management. The process is still in progress and I don't know yet what it will provide at the end. It just looks good for the moment.
I don't say that you should postpone what you have planned but I'll have more on this at the end of the day. I think that both efforts should be considered worthwhile. I wanted to tell you that if you stumble upon some overwhelming task ahead, you should know that some alternative may bring another way around the issue.
Will remain to provide for some solution to the MISSING search tool issue...