(I did this in ts2010, still works with tfs2012)
add to your msbuild .proj, something like this
<Project DefaultTargets="SetEnv" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="3.5">
<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\TeamBuild\Microsoft.TeamFoundation.Build.targets" />
<PropertyGroup>
<TeamBuildRefPath Condition="'$(TeamBuildRefPath)'==''">$(PathToMyDevenv)\IDE</TeamBuildRefPath>
</PropertyGroup>
then the $(DropLocation) becomes populated as you'd expect.
I've a feeling you only need the import, it's been a while since I did this.
Cheers