One could run a filter function on bb_template to check for templates in both the parent and child themes.
Or, the user could set it in bb-config.php:
define( BB_DEFAULT_THEME_DIR, 'path/to/default/theme/directory' );
I don’t really like either of those two options. Your average user gets confused enough as it is. Trying to tell them to input PHP code can cause headaches sometimes.
As far as I know, there’s no way to filter the default theme directory either. I’m open to ideas though. If anyone could offer a solution with a few lines of code, I’ll be happy to give it a whirl.
Ideally, the constant BB_DEFAULT_THEME_DIR would be based off the theme’s Template line in style.css. Or, bb_get_default_template() would recognize the Template line.
Strategically the most important thing we need to figure out is how to integrate bbPress better with WP more for people who want that — right now it’s easier to use one of the WP plugins for forums than bbPress.
You have told it yourself, just turn bbpress into a wppress plugin. Actually, you can turn wppress into a forum with plugins like tdo-mini-forms. So, you could be including a forum with the next version of wp in a matter of minutes with a couple of silly tweaks. I’ve been on the point of doing this myself several times, I don’t know why I keep on struggling with bbpress integration.
If you’d like to influence the future of bbPress, please drop in on this thread:
http://bbpress.org/forums/topic/help-out-with-bbpress