Silent install of SQL Server 2008r2 for ALL users?

Question

We ship an installer that does a silent install of SQL Server 2008r2 with all the 'right' parameters pre-configured. We use this:
/QS /INSTANCENAME=MSSQLSERVER /ACTION=Install /FEATURES=SQL,BC,Conn,ADV_SSMS /SAPWD=MyPasswordGoesHere /SECURITYMODE=SQL /TCPENABLED=1 /ADDCURRENTUSERASSQLADMIN /SQLSVCSTARTUPTYPE=Automatic /SQLSVCACCOUNT="NT AUTHORITY\SYSTEM" /AGTSVCACCOUNT="NT
AUTHORITY\SYSTEM" /IACCEPTSQLSERVERLICENSETERMS
However, the problem is that if a tech installs it for another user, that user does not have access, since ADDCURRENTUSERASSQLADMIN flag only adds the user who installed it -- rather than adding ALL users on the computer. (Or at least all admins.)
I tried replacing ADDCURRENTUSERASSQLADMIN with this:
/SQLSYSADMINACCOUNTS="BUILTIN\ADMINISTRATORS"
but that also failed to work. How can we install for all users, or at least all administrators?
NOTE: we do NOT know the name of the computer or the users. The installer has to work for any computer setup.
Similar question has already been asked, but wasn't helpful:
http://social.msdn.microsoft.com/Forums/sqlserver/en-US/8354675f-be9b-4909-b776-942f03954807/how-to-do-a-silent-install-of-sql-2010-express-for-all-users-on-a-computer?forum=sqlexpress
The documentation does not make it clear what you can pass as allowed SQLSYSADMINACCOUNTS parameters. I also am not sure if I would need to change the AGTSVCACCOUNT or SQLSVCACCOUNT?

Answer 1

I tried one more thing:
/SQLSYSADMINACCOUNTS="BUILTIN\Administrators"
which does not halt installation with an error. 
However, doing:
osql -E
create database test
go
in a command prompt from a different admin user on the same machine results in a permissions error... but works if done from a command prompt on the user account from which MSSQLServer was installed.
So for some reason /SQLSYSADMINACCOUNTS="BUILTIN\Administrators" does not work as expected.
Anyone have any idea why?
I checked the output log from the install, and found it had generated and used a configurationFIle to do the install. I looked in that file, and these were the settings, even though I did NOT pass /ADDCURRENTUSERASSQLADMIN as a command line option:
; Windows account(s) to provision as SQL Server system administrators. 
SQLSYSADMINACCOUNTS="BUILTIN\Administrators"
; The default is Windows Authentication. Use "SQL" for Mixed Mode Authentication. 
SECURITYMODE="SQL"
; Provision current user as a Database Engine system administrator for SQL Server 2008 R2 Express. 
ADDCURRENTUSERASSQLADMIN="True"