I've made a temporary fix.
In /hashbuddy/includes/hashbuddy.php I switched out line 69 for this:
$content = preg_replace( $pattern, ' <a href="'. $site_url . '/activity/?s=%23' . htmlspecialchars( $hashtag ) . '" rel="nofollow" class="hashtag" id="' . htmlspecialchars( $hashtag ) . '">#'. htmlspecialchars( $hashtag ) .'</a>', $content );
I don't know how to call the activity root of Buddypress in that php link, so I put it in raw before "/>s=#23", as that's my activity root.
The temporary fix searches sitewide activity and displays the BP Activity items bbPress generates. Not ideal, but it will do for now.