Post Build events are too late in the game. You have found this out to an extent, as I did, by realizing you need to alter the file in obj.
However, Post Build occurs after the manifest generation. So editing the obj file there is too late. The better place to do it is in the AfterCompile build target.
You'll have to edit the csproj; add after the lines you'll already see for this:
<!-- To modify your build process, add your task inside one of the targets below and uncomment it.
Other similar extension points exist, see Microsoft.Common.targets.
<Target Name="BeforeBuild">
</Target>
<Target Name="AfterBuild">
</Target>
-->
**<Target Name="AfterCompile">
<Exec Command="
call "$(VS110COMNTOOLS)\vsvars32.bat"
editbin /largeaddressaware "$(IntermediateOutputPath)$(TargetFileName)"
" />
</Target>**