This thread is almost entirely for my benefit. It is mostly to allow me to detail my progress, but anyone can post if they want to. I want to create an add-on, but I have been reluctant to do so because I'm concerned the idea is not very appealing.
The general idea of the add-on would be to enable users to view threads from more than one node, on a single page, or 'forum_view'. Nodes which are selected as "added to Multiview"' would be viewable from a separate page. This page would be accessible through a link added to the navigation bar.
It might be useful to users who are much more active on only a handful of boards, but less so on others. Having this option would stop them having to navigate back-and-forth between boards, and would focus their attention within only the boards they are interested in. It would all be accessible on only one page.
I have already made some progress with it.
Actually, now I've typed all of that, it doesn't seem like such a good idea after all.