I looked into this in the past and having read a number of existing threads on the topic it looks like there is no way to change this.
For aesthetic reasons I chose to change from the default as “/forum/forums/” doesn’t look great so you can change it to “/bb/forums” or “/forum/discuss/” or whatever. Not ideal but trying to hack this to work could leave you in a whole world of pain (prob tricky to get right, and may break with future releases).
Did you end up finding a solution to this? I’m trying to do the exact same thing.
I have found a few threads from 4 or 5 years ago, and supposedly someone wrote a plugin for an old version of bbpress that can do this. But BBpress has gone through some serious changes (for the best) and now operates as a plugin for wordpress not a stand alone platform – so they no longer work.
Does anyone know of a plugin/rewrite rules/functions.php code we can use to make this a reality?