Originally posted by Adam One feasible way to do it, that I see, is as follows:
-User-defined "custom" forums in the user cp, either an unlimited number, or a fixed number
-forumdisplay.php?f=custom_x as the access parameter
-When the custom parameter is passed, the query inside forumdisplay pulls threads from all forums defined in the user's custom forum option for that particular x.
Doesn't seem that bad, I don't think, but I can see it taking a good 12 hours to implement.
Adam, great to see that you may have found a way.
I can see traps in the details, so, let me address them first.
1. The feature may be difficult to access for a user. A predefined custom forum for the set of first four subforums now under "Pentax Cameras" may solve this, accessible from the Forum homepage in the same Pentax Cameras category. After all, we want to counteract the "every topic is started four times" effect, so it must be easy. And it must be named properly too. So that everyone knows that visiting this custom forum saves a visit to each of the other four.
2. The listing of threads should be the same or very similiar, e.g., ordered by last modification. Just replacing the "new thread" button for a button leading to a selection of the four individual forums. So, it would just be a flat view.
So yes, it would look a lot like the tag search result page, except for the new thread button and the page being accessible as a sibling to "Pentax DSLR discussion" w/o user intervention.