Hi Diana
I can only think of using the XREF1, XREF2 or XREF3 fields in that case. I would create a substitution rule which will populate ZUONR value in one of these fields and than the users can do a mass update in FBL* reports, which is a standard process. Now in OB74 you can set up clearing rules to run on these fields and schedule F.13 in the background for automatic clearing. This should take care of not touching ZUONR values at all and also look at automatic clearing as you want.
Thanks & regards
Sanil Bhandari