After looking through the WiX source code, I found the properties. The one I decided to use was simply NETFRAMEWORK45. The others can be found here: http://wix.codeplex.com/SourceControl/changeset/view/62deafa7b2a4#src/ext/NetFxExtension/wixlib/NetFx4.5.wxs
UPDATE: Moved to https://github.com/wixtoolset/wix4/blob/709560017c322dca0059d1c8849184e22b85bea7/src/ext/NetFxExtension/wixlib/NetFx4.5.wxs