Get custom field values into the sidebar/widget?

Definitely doable with tons of different ways to accomplish it. Basically, you can use a widget that allows for the execution of PHP or modify your theme files to execute PHP in the sidebar. The main function you will need is get_post_meta. You also need to access the post/page/CPT ID. You can use code like the following to get your post meta displayed.

global $wp_query;
if(is_object($wp_query->queried_object) && $wp_query->queried_object->ID)
{
    echo get_post_meta($wp_query->queried_object->ID, 'my_post_meta_key', true);
}

Obviously, you need to change the “my_post_meta_key” value to a key that corresponds to the meta data that you want. Also, there are other ways to access the current page/post/CPT ID, but since I do not know your specific context, this will work for most situations.