You may have answered your own question. If the S/N or another asset tag has not been assigned, then I doubt you can get it that way. On the other hand, if Dell does that as a part of their manufacturing process, then it should be there (are you SURE there is no ID in the CMOS). If you can get access to the CMOS setup, then you should be able to find it there. A utility that can read CMOS data should be able to get it for you. I don't know how old the unit in question is so I don't know ifthat information is included or not. Your best bet would be to contact Dell technical support for assistance.
If this is a one time thing, you may just have to contact the user and have them read the S/N off of the back panel or other location that Dell may have put the S/N.