Is it possible to write an extension for Visual Studio 2010 that provides syntax highlighting, intellisense, outlining, etc for a custom template or markup language
you can write your own editor extension, see this article : http://www.devx.com/VS_2010/Article/45058
And to understand how it works : http://msdn.microsoft.com/en-us/library/dd885240(v=VS.100).aspx
Do research about "Managed Extensibility Framework (MEF) "
You can download an exemple for the Ook langage here
You should take a look at https://github.com/SparkViewEngine/SparkSense as an example. It was built for the spark view engine to provide intellisense for VS 2010.
"SparkSense is a plugin for Visual Studio 2010 that enables various tooling support and productivity features when using Spark as a View Engine for various MVC frameworks including ASP.NET MVC"