Here’s an idea for you:
If you create a script for importing the content from Drupal, using the node ID as a suggested ID for the new WP post (Otto has explained how to do this here: How can I assign post a specific ID on creation?), this would lead to your nodes being accessible in the new site on https://yoursite.com/?p=XXX for each of them, which will be translated to the new URL structure. Then setting up a plugin like https://wordpress.org/plugins/redirection/ for a regex redirection of any request that comes for /node/XXX to be redirected to /?p=XXX you’ll have what you are looking for without having to create custom fields. Beware that this would work if your website is clean of other posts at the moment of migrating your Drupal content, as the ID is added only as a suggestion and will not be the same, if it already exists in wp_posts table.
I’m pretty sure it would be faster to migrate the Drupal content programatically rather than having people to manually do this and then set the extra field that you speak of.
If you insist on having your content manually transferred or you can’t go with my first suggestion, then you could go for another approach – intercepting the 404 handling in WordPress by using pre_handle_404 or template_redirect filters could allow you to parse the request for the node ID if it matches the pattern siteurl/node/XXX, find your right post with the sought ACF value (WP_Query with a meta_query in it) and redirect to the page that you want your visitors to end up on.