We discovered on our own that you have to hack it.
With AutoCAD 2008 installed, run each object enabler .exe that you need, in turn. Select "AutoCAD 2008" as the sole target for installation. Just before clicking "Finish" in the wizard, copy the .msi file in created in your TEMP folder to a better location, and rename it to something like civil2008oe_AutoCAD.msi
For the Civil, MEP, and Architecture object enablers (but not MDT or Raster Design), do the following: Run Microsoft Orca (free basic .msi editor) to edit your new copy of the .msi file. In Orca, in the Directory table, in the line for INSTALLDIR, change the "Directory Parent" from TARGETDIR to ProgramFilesFolder, and change the "DefaultDir" from enabler to AutoCAD 2008, then save the edited .msi file.
Your new .msi's can be installed silently via group policy.
If you have additional AutoCAD products besides base AutoCAD, you'd need to do the same process, creating a new set of MSIs, for each product.