In WP-Syntax, just change the part at the end:
// Add styling
add_action('wp_head', 'wp_syntax_head');
// We want to run before other filters; hence, a priority of 0 was chosen.
// The lower the number, the higher the priority. 10 is the default and
// several formatting filters run at or around 6.
add_filter('the_content', 'wp_syntax_before_filter', 0);
add_filter('the_excerpt', 'wp_syntax_before_filter', 0);
add_filter('comment_text', 'wp_syntax_before_filter', 0);
// We want to run after other filters; hence, a priority of 99.
add_filter('the_content', 'wp_syntax_after_filter', 99);
add_filter('the_excerpt', 'wp_syntax_after_filter', 99);
add_filter('comment_text', 'wp_syntax_after_filter', 99);
to:
// Add styling
add_action('bb_head', 'wp_syntax_head');
// We want to run before other filters; hence, a priority of 0 was chosen.
// The lower the number, the higher the priority. 10 is the default and
// several formatting filters run at or around 6.
add_filter('post_text', 'wp_syntax_before_filter', 0);
// We want to run after other filters; hence, a priority of 99.
add_filter('post_text', 'wp_syntax_after_filter', 99);
And it should work in bbPress.
The google syntax highlighter code seems huge with too many externals so I don't recommend it. Even Geshi is nearly 200k of PHP code which is insane but still faster than Google's javascript.
I won't be running it here until I find a lighter solution.