Service "Interceptor" would be a nice touch.
In a similar but now discontinued Agatha RRSL service (source available on git), there's a "interceptor". It has constructor that allows IoC, BeforeHandling, AfterHandling, plus RequestContext that has access to both Request and Response object. You can register multiple interceptors at start up and they will wrap around each other, and around the default handler. Would be nice feature that allows true aspect programming in ServiceStack.