BuddyPress.org

I figured out I could intercept it earlier.

bp_core_screen_signup is loaded in the action bp_screens with a default priority of 10, so I added


add_action( 'bp_screens', 'bp_core_screen_signup_tweak', 9 );
function bp_core_screen_signup_tweak() {
if ( isset( $_POST['signup_password'] ) ) {
$_POST['signup_password_confirm'] = $_POST['signup_password'];
}
}