@Unique uses part of the users name and a part that is generated based on the time. So as long as your unique numbers are generated by different users they will 99.9% be unique irrelevant whether you have access to it or not.
If your unique number is generated by an agent and therefore by one id and if you are generating alot you might run into trouble.
See this post from Andre Guirard (well the whole thread might be of interest to you):
http://www-10.lotus.com/ldd/nd6forum.nsf/55c38d716d632d9b8525689b005ba1c0/9845c7150d19a26b8525713b0050842b?OpenDocument