@tnolte there might be two different issues here, so I’ll see if I can speak to both of them.
1. you’re right, you shouldn’t put the ID into a custom field. It might be a problem that it isn’t working, I’m not really sure about that and you might need to do more logging to find out why it isn’t, but it’s definitely not necessary since the object map connects the post ID and the Salesforce record ID. If there is a problem, it might be because WordPress doesn’t create custom fields (or any meta fields) until the object has already been created. I don’t think that would keep it from using a custom field as a prematch field; I think I’ve tested this in the past, but I’m not positive. So it would be good to learn more about this.
2. There is indeed an object map box on the WordPress user edit screen, but there isn’t one on any other object types. There is not much intentionality to this, it’s unfortunately just because the user screen is the main one that isn’t potentially affected by external factors (Gutenberg, for example). There’s a longstanding GitHub issue to make a box for other object types. I’d happily review pull requests for a box on other WordPress objects, but I don’t have a current plan to work on that.