First, I very much agree with Paul's comment - the only reason I saw this post was because I just happened to see it in the Recent Posts column on the right side of the page just now. I don't know that I have knowingly ever perused the "Tech Support for the OGR Forum (not for trains)" subforum (all due respect). What you are proposing to do is a pretty big deal, seems to me, so it ought to get advertised in all the fora and sub-fora.
I appreciate the effort to try to keep the various fora "pure" - as I'm sure most readers do. I have a couple of suggestions that might improve posting accuracy (maybe you've already thought about them ...). You might consider changing the "post reply" button label to repeat the forum name -- so in this case it would read "post reply to Tech Support for ..." Well, ok, that's probably too long, but there might be some short hand labels to use. Seems to me for posters using computers the real estate required for a bigger/longer button isn't much of an issue. Probably is for posters using smart phones. If its not possible to pick up the actual forum name, maybe just a general 'warning label' to alert the poster to checking to make sure its to the forum he/she intended.
A second, similar approach would be to have a two-step posting process. the first step is a "preview" (purgatory) step (a couple of non-train fora that I read and post to have two-step processes) and then a "post" or "submit" step. The preview/purgatory step could then offer the feedback of saying something like, 'this posting will go to the ZZZZ forum ... Do you want to change the destination forum?' Likely some extra coding involved in doing this, but it could save you some of the forum topic purity policing headaches.
Is this an issue for new posts or just for reply posts? I would have thought that the steps to set up a new post would make most posters think about which forum they are targeting. Would it be possible to cross-post to one other forum? The separation between some of the forum topics is ambiguous (a good example is the Electrical sub-forum and any one of the Control System sub-fora - another is the Photo Albums sub-forum and almost any other forum).
All of the above notwithstanding, the OGR forum is one of the most easily navigated sites around and fairly accessible by various topics that don't require you to dig very deep. I wish some of the woodworking fora that I occasionally tune in to were as transparent and as easy to find topics and read as OGR. Nice job.