I had the same problem. Or, nearly. Simplepress doesn’t work with 404 Redirected too. So i dive into the code in the include/frontend.php and addet 2 lines:
In function wbz404_process404() after the line 63 and in function wbz404_redirectCanonical() after the line 187 (in both cases directly after $requestedURL .= wbz404_SortQuery($urlParts);):
/* hack, dont redirect forum links */
if (preg_match("#\/forum\/#si",$requestedURL) == 1) return null;
You have to replace the regex to your needs, but this works for all urls with this scheme. And if I mean all, I mean really all. in this case the path /tag/forum/ will also never be redirected. Or /blog/category/forum/blabla.
So, for me it’s okay, but there is some potential for optimizing. Too bad, that this plugin wasn’t updated sice 2 years…