Hi Joshua,
The second solution you mentioned is possible for setting up a custom report so that when your users go to pick a Related Item they are only shown Items that haven't already been picked. I have done this before when a builder wanted unique ids for training events.
You can do this by creating a summary field between your two tables so that you summarize the number of records that are related to each Item. With that summary field you can then make a report on the Items table that only shows Items that have <1 related records. This way you are only shown unassigned items on the report. Then you can set this report on your form for the Related item field, this way once a user selects an item and saves the record it will disappear off the report but later on if necessary you can find it as the Admin and even add more related records if you had a need to. It does require the user to pick a related Item and if they use a Grid Edit you would want to make sure to set your reports to use the same form to prevent them from getting around the report filtering. I hope that is helpful.
