Can I modify package.xml file in SQL bootstrapper to install a named SQL server instance

I want to use the SqlExpress2008 Bootstrapper for a new installation on Windows7, I do not want to use the default SQLEXPRESS Instance.

I have attempted to edit the package.xml file located in: C:\Program Files\Microsoft

SDKs\Windows\v7.0A\Bootstrapper\Packages\SqlExpress2008\en\package.xml

and updated the command argument instancename=CUSTOMINSTANCE

    <Command PackageFile="SQLEXPR32_x86_ENU.EXE"
             Arguments='/q /hideconsole /action=Install /features=SQL /instancename="CUSTOMINSTANCE" /enableranu=1 /sqlsvcaccount="NT Authority\Network Service" /AddCurrentUserAsSqlAdmin /skiprules=RebootRequiredCheck'
             EstimatedInstalledBytes="225000000"
             EstimatedInstallSeconds="420">

But unfortunately it still creates the default SQLEXPRESS not CUSTOMINSTANCE

The wix tag is as follows:

   <sql:SqlDatabase
              Id="SqlDatabaseCore"
              ConfirmOverwrite="yes"
              ContinueOnError="no"
              CreateOnInstall="yes"
              CreateOnReinstall="no"
              CreateOnUninstall="no"
              Database="MyDatabase"
              DropOnInstall="no"
              DropOnReinstall="no"
              DropOnUninstall="no"
              Instance="[SQLINSTANCE]"
              Server="[SQLSERVER]">
              <sql:SqlFileSpec
                Id="SqlFileSpecCore"
                Filename="[CommonAppDataFolder]MyCompany\Database\MyDatabase.mdf"
                Name="MyDatabase" />
              <sql:SqlLogFileSpec
                Id="SqlLogFileSpecCore"
                Filename="[CommonAppDataFolder]MyCompany\Database\MyDatabase.ldf"
                Name="MyDatabaseLog" />

<Property Id='SQLSERVER'>.</Property>
<Property Id='SQLINSTANCE'>CUSTOMINSTANCE</Property>

Is this the standard way to accomplish this?

I don't know about a standard but we passed the new instancename to the SQLExpress setup executable on the command line. First, decompress the SQLEXPRADV_x86_ENU.exe file into a temp directory,

string workingDir = Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location);

string pathToSQLSetupTempDirectory = workingDir + Path.DirectorySeparatorChar + "sqlsetup";

if (!System.IO.Directory.Exists(pathToSQLSetupTempDirectory))
{
System.IO.Directory.CreateDirectory(pathToSQLSetupTempDirectory);
}

string path = Path.GetFullPath(workingDir + Path.DirectorySeparatorChar + "SQLEXPRADV_x86_ENU.exe");
string args = "/Q /X:" + pathToSQLSetupTempDirectory;

Then, construct the command line parameters and run the extracted setup.exe file. To prevent installation of the management tools, add "/Features=SQL" to the command line arguments:

 path = pathToSQLSetupTempDirectory + Path.DirectorySeparatorChar + "Setup.exe";
 args = "/QUIET /Action=Install /Features=SQL,Tools /InstanceName=" + instanceName +
    " /SECURITYMODE=SQL /TCPENABLED=1 /SAPWD=" + sqlSAPswd + " /SQLSYSADMINACCOUNTS=\"Builtin\\Administrators\" /SQLSVCACCOUNT=\"NT AUTHORITY\\NETWORK SERVICE\"";

http://msdn.microsoft.com/en-us/library/ms144259.aspx has more about the supported arguments.