Old thread, I know, but I found a way to do this that I keep forgetting, and on my search to find it one last time, I stumbled upon this question. The best way I’ve found to this is is to use the BeforeBuild target in the .csproj file.
<Target Name="BeforeBuild">
<ItemGroup>
<Content Include="**\*.less" />
</ItemGroup>
</Target>
VS 2010 won’t touch this section, and it ensures that your files are included as content every time the project is built.