I found a script to slipstream SP1 on Fran's blog here:
Quick way to create a SQL Server 2008 R2 installer with integrated SP1 (Slipstream)
But it was dealing with SP1. So, a quick "replace" in Notepad++, and hey presto, it works for SP2 as well. here's the batch file:
@ECHO OFF
::File Verification
IF NOT EXIST "C:\SQLServer2008R2_SP2\" GOTO ERROR
IF NOT EXIST "C:\SQLServer2008R2SP2-KB2630458-x86-ENU.exe" GOTO ERROR
IF NOT EXIST "C:\SQLServer2008R2SP2-KB2630458-x64-ENU.exe" GOTO ERROR
IF NOT EXIST "C:\SQLServer2008R2SP2-KB2630458-IA64-ENU.exe" GOTO ERROR
::Extract each of the SQL Server 2008 R2 SP1 packages to C:\SQLServer2008R2_SP2\SP
ECHO Extracting SP files to C:\SQLServer2008R2_SP2\SP...
start /wait C:\SQLServer2008R2SP2-KB2630458-x86-ENU.exe /x:C:\SQLServer2008R2_SP2\SP
ECHO Extract of SQLServer2008R2SP2-KB2630458-x86-ENU.exe Completed
start /wait C:\SQLServer2008R2SP2-KB2630458-x64-ENU.exe /x:C:\SQLServer2008R2_SP2\SP
ECHO Extract of SQLServer2008R2SP2-KB2630458-x64-ENU.exe Completed
start /wait C:\SQLServer2008R2SP2-KB2630458-IA64-ENU.exe /x:C:\SQLServer2008R2_SP2\SP
ECHO Extract of SQLServer2008R2SP2-KB2630458-IA64-ENU.exe Completed
ECHO.
::Copy Setup.exe from the SP extracted location to the original source media location
ECHO Copying Setup.exe to C:\SQLServer2008R2_SP2
start /wait robocopy C:\SQLServer2008R2_SP2\SP C:\SQLServer2008R2_SP2 Setup.exe
ECHO Setup.exe copied
ECHO.
::Copy all files, not the folders, except the Microsoft.SQL.Chainer.PackageData.dll, in C:\SQLServer2008R2_SP2\SP\<architecture> to C:\SQLServer2008R2_SP2\<architecture> to update the original files
ECHO Copying files to C:\SQLServer2008R2_SP2\
start /wait robocopy C:\SQLServer2008R2_SP2\SP\x86 C:\SQLServer2008R2_SP2\x86 /XF Microsoft.SQL.Chainer.PackageData.dll
ECHO Files from C:\SQLServer2008R2_SP2\SP\x86 copied
start /wait robocopy C:\SQLServer2008R2_SP2\SP\x64 C:\SQLServer2008R2_SP2\x64 /XF Microsoft.SQL.Chainer.PackageData.dll
ECHO Files from C:\SQLServer2008R2_SP2\SP\x64 copied
start /wait robocopy C:\SQLServer2008R2_SP2\SP\ia64 C:\SQLServer2008R2_SP2\ia64 /XF Microsoft.SQL.Chainer.PackageData.dll
ECHO Files from C:\SQLServer2008R2_SP2\SP\ia64 copied
ECHO.
::Create or modify DefaultSetup.ini
IF NOT EXIST "C:\SQLServer2008R2_SP2\x86\DefaultSetup.ini" GOTO CREATEINI
ECHO Modifying DefaultSetup.ini
TYPE C:\SQLServer2008R2_SP2\x86\DefaultSetup.ini > C:\SQLServer2008R2_SP2\x86\Temp_DefaultSetup.ini
ECHO PCUSOURCE=".\SP" >> C:\SQLServer2008R2_SP2\x86\Temp_DefaultSetup.ini
DEL C:\SQLServer2008R2_SP2\x86\DefaultSetup.ini
REN C:\SQLServer2008R2_SP2\x86\Temp_DefaultSetup.ini DefaultSetup.ini
ECHO.
GOTO COPYINI
:CREATEINI
ECHO Creating or modifying DefaultSetup.ini
ECHO ;SQLSERVER2008 R2 Configuration File > C:\SQLServer2008R2_SP2\x86\DefaultSetup.ini
ECHO [SQLSERVER2008] >> C:\SQLServer2008R2_SP2\x86\DefaultSetup.ini
ECHO PCUSOURCE=".\SP" >> C:\SQLServer2008R2_SP2\x86\DefaultSetup.ini
ECHO.
:COPYINI
::Copy DefaultSetup.ini from x86 to x64 and ia64 folders
ECHO Copying DefaultSetup.ini from x86 to x64 and ia64 folders
start /wait robocopy C:\SQLServer2008R2_SP2\x86 C:\SQLServer2008R2_SP2\x64 DefaultSetup.ini
start /wait robocopy C:\SQLServer2008R2_SP2\x86 C:\SQLServer2008R2_SP2\ia64 DefaultSetup.ini
ECHO.
ECHO ==============================================================================
ECHO OPERATION COMPLETED
ECHO Use C:\SQLServer2008R2_SP2\Setup.exe to install SQL Server 2008 R2 + SP2
ECHO ==============================================================================
ECHO.
GOTO FIN
:ERROR
ECHO ==============================================================================
ECHO FILE VERIFICATION FAIL, PLEASE CHECK THE FOLLOWING:
ECHO THE DIRECTORY C:\SQLServer2008R2_SP2\ EXISTS
ECHO SQLServer2008R2SP2-KB2630458-x86-ENU.exe FILE IS LOCATED IN C:\
ECHO SQLServer2008R2SP2-KB2630458-x64-ENU.exe FILE IS LOCATED IN C:\
ECHO SQLServer2008R2SP2-KB2630458-IA64-ENU.exe FILE IS LOCATED IN C:\
ECHO ==============================================================================
ECHO.
:FIN
PAUSE
Thanks to Fran Lens for the heavy lifting.