Use this sample to learn to write custom code to enhance languages developed with the DSL Tools. This download includes a detailed guide to implementing customized features, together with complete sample solutions based on the Activity Diagrams, Class Diagrams, and Use Case Diagrams templates. Features demonstrated include: computed properties, constraints on connections, line routing, shadow & color gradient control, shape constraints, and template-generated code.
Alan has just posted a whole bunch of code customization samples to work against the September CTP of the DSL Tools.
These give you a flavour of a few things we'd expect our API to enable (although the API itself will change plenty before we ship V1).
To quote the blurb: