Hello Nick,
Yes, you can update the $_POST value when validating the form. For example:
add_filter( 'rwmb_frontend_validate', function( $validate, $config ) {
$_POST['field_id'] = 'test field value';
rwmb_request()->set_post_data( $_POST );
return $validate;
}, 10, 2 );
Following the documentation https://docs.metabox.io/actions/rwmb-before-save-post/