This seems an inelegant way to achieve this result. Bear with me a moment.

The problem is that you want forum rules easily accessed from within each forum, but you would like to not have to update them individually, nor worry about them getting out of sinc. I.E., you want to be able to edit those rules once and have them update globally.

Using a pinned post seems a reasonable way to solve this problem mostly because most forum software could do this (though without globally updating). Using pinned posts seems inelegant to me, since I presume it can't be easily implemented without some fairly odd hack. I would assume you'd need to split off forum location into a junction table or perhaps, add logic to look up the 'multi-forum threads' and place them at top.

Mightn't  it be more elegant to think of forum rules instead as help files and add a system to add customized help files to the forum? For instance, in some forums you might want to have a link on how to post, or how to format. The forums currently display at top of the page, 'My Settings', 'My Posts', 'My Threads', 'Search', etc. Allowing people to customize that area and apply CSS seems like it could address the problem above, others not raised, and actually give DNN Forums a new feature, rare in forum software.

As a hack to your problem Vin,  you could consider keeping your rules elsewhere, making individual forum posts which display the external webpage. Then you'd only have to update it in one place. It is hacky, but you could do it today.