Assuming a contentproj is just a collection of files, what you can do is add the harvesting directly within the wixproj that is creating the installer:
<PropertyGroup>
<HarvestDirectoryNoLogo>true</HarvestDirectoryNoLogo>
<HarvestDirectorySuppressFragments>true</HarvestDirectorySuppressFragments>
<HarvestDirectorySuppressUniqueIds>true</HarvestDirectorySuppressUniqueIds>
<HarvestDirectoryAutogenerateGuids>true</HarvestDirectoryAutogenerateGuids>
</PropertyGroup>
<ItemGroup>
<HarvestDirectory Condition=" '$(Configuration)|$(Platform)' == 'Debug|x86' "
Include="$(SolutionDir)\GameContent">
<DirectoryRefId>INSTALLDIR</DirectoryRefId>
<SuppressRootDirectory>true</SuppressRootDirectory>
<PreprocessorVariable>var.GameContentDir</PreprocessorVariable>
<ComponentGroupName>GameContent</ComponentGroupName>
</HarvestDirectory>
</ItemGroup>
You will need to add this manually to the wixproj file and you can repeat the HarvestDirectory for each directory if you require more than one.
To set the var.GameContentDir
pre-processor variable edit the DefineConstants property:
<DefineConstants>GameContentDir=$(GameContentDir);</DefineConstants>
which will set the pre-processor var to the msbuild property:
<GameContentDir>$(SolutionDir)\GameContent</GameContentDir>
which means you can then modify this dependant on the build configuration. If you don't need to modify the path, just set a static value in the <DefineConstants>
property.
This will then generate a wxs file in the obj directory each build which is then included assuming you have included the ComponentGroupName. If you have included the one you previously generated in your wixproj remove it as you will get conflicts if the ComponentGroupName is the same.