[Mark]

It’s actually a pretty simple hack, though I caution you that I worked it out in the last five minutes.

First, modify the following three lines of your bbpress .htaccess;

RewriteRule ^forum/([^/]+)/page/([0-9]+)/?$ /bbpress/forum.php?id=$1&page=$2 [L,QSA]

RewriteRule ^forum/([^/]+)/?$ /bbpress/forum.php?id=$1 [L,QSA]

RewriteRule ^forum/?$ /bbpress/ [R=302,L,QSA]

change ^forum/ to whatever you want the permalink to be, such as ^sub/ or ^purplepolkadots/, whatever works for you. Then open functions.bb-template.php and go to line 711 inside function get_forum_link() (line numbers may vary). It should look something like:

$link = bb_get_uri('forum/' . $forum->$column . $page, null, $context);

…and change ‘forum/’ to whatever you put in your .htaccess – this seemed to work for me immediately, however as I mentioned, I haven’t done any thorough testing of it.