I used to be a FoxPro programmer 16 years back. I have not looked back since using FileMaker Pro. Definitely less powerful... much more approachable.
The unique ID you are proposing is very limiting. You have a potential of 67 600 combinations, a small amount for any solution that hopes to have a long life. If, on top of that, you need to change it as you suggest is a possibility, then I do not envy your task at that time. There will be related records that will also need changing.
May I suggest you make a hidden unique ID? You can use a simple numeric serial number. On the other hand, you could use an alphanumeric serial number like I do. This can be handy to add context to a key.
As for your need to add a visible unique ID:
1. You will need a calculation field that extracts the first two characters of a person's last name.
2. You will need a self-join relationship that links from the person's record, using the calculated two character identifier as a key field, to the same table using the same field.
3. You will need to attach a OnObjectKeystroke script trigger to the last name field.
4. The target field will need to look at the number of characters and, when there are two characters or more, look up the existing count of similarly named record to calculate the numeric portion of the key. The script would then complete the visible key, commit the record, then place the cursor back to the same position in the last name field before the script was triggered.
Hope this helps!