>I've been using sys(2015) as the field default for certain unique keys. Am I asking for trouble? I'm not sure how sys(2015) generates the string and for how long I can expect to keep getting unique strings..
>Any other ideas for unique keys?

Yes, some people are using that technique.

However, the more used one is by using a data dictionary table that hold the actual key of each table and some others used fields. By having that table, you can also apply a locking mecanism which will garantee to always have a unique key.