.NET plugin or script engine

I need a .NET expert who can help me integrating an (existing) script engine or plugin framework in an ASP.NET web application. I have setup a very simple type of workflow engine (with sometimes long-running workflows), which connects tasks/input-forms. However sometimes routes needed to be conditional, e.g. a condition needs to be met before the next task is presented. Since the workflows integrate with a ASP.NET web application I need a flexible script engine, so I can add business rules at any time without the need to recompile the application. Ideally some type of plugin framework can be used so expressions can be done in C# or VB.NET code, but alternatively I can imagine using an script engine like Jint (on codeplex) or JScript.net. Please mention your expertise and experience.