Silent install of SQL Server 2008r2 for ALL users?

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 compute ...