I do believe that is possible, in fact it is exactly what I do on my vb 3.8 forum with this mod installed...
You need a collection of icons that have file names like 2.png, 3.png, 4.png, etc... basically ForumID#.png for each forum id you have...in this example in your misc images directory but you could make it anywhere you want.
Then manually edit the threadbit template... Find a suitable location and add the code:
Code:
<img src="images/misc/{vb raw thread.forumid}.png" border="0" />
I'm not able to test it right now but I think it should work.
You can add a conditional too so if the $thread[forumid] equals $forum[forumid] you don't show any icon at all. (Because the thread is from the same forum you are viewing).