Even i have similar requirement like your.
decided for screen customization where user field is actually updated using custom program.
will update you details as testing is going to happen from Monday on wards.
the logic for us was we mapped it to email id .
since my ldap is integrated manyl dap server employees from 5 company .
and their sap id is also different even though they logon to same server.
1)for comapny 1 sapid is S_lastname+1
2)Company2 Lastname+1(numeric)
3)Company 3 Lastname+p
4)Company4 Lastname+fristdigit of 1stname
5)Company5 lastname+1(numeric)+P
suggested to have uniform naming convention through out landscape but that going to happen next year.
earlier they use to create id in GRC 5.3 system and it was not connected to LDAP.
so user id was populated automatically.
Regards,
Prasant