I have a TFS environment with the following branching setup
You'll have to do some trickery...
<configuration>
<packageSources>
<add key="Branch X packages" value="http://www.myget.org/F/corpxyzbranchxyz" />
</packageSources>
<disabledPackageSources />
<activePackageSource>
<add key="Branch X packages" value="http://www.myget.org/F/corpxyzbranchxyz" />
</activePackageSource>
</configuration>
This does mean that you'll have to have a different nuget.config file per branch and that you should be careful when merging/branching.