Actually this turned out to be a lot easier than I thought it would be, but since I could find NO documentation on this hook anywhere, figured I'd add something here.
In my case, there is a post type from another plugin that I simply do not want to display as it only confuses the end user...
Accomplishing this cleanly was rather easy using 2 action hooks. Code sample follows:
We'll call the target post_type "foo"
add_action('registered_post_type', 'my_registered_post_type_handler', 10, 2);
function my_registered_post_type_handler($post_type, $args) {
do_action("my_registered_{$post_type}_post_type", $post_type, $args);
}
add_action('my_registered_foo_post_type', 'my_registered_foo_post_type_handler', 10, 2);
function my_registered_foo_post_type_handler($post_type, $args) {
remove_action(current_filter(), __FUNCTION__, 10, 2); // only run once
// change your args here
$args['show_ui'] = false;
// re-register
register_post_type($post_type, $args); // will call this hook again if it exists, so we removed it above.
}
And we are done.