Scott Allen pointed out that if you need to declare top level properties in a XAML only workflow, you can subclass the root activity class, add the properties you want, then specify the custom subclass in the XAML workflow. That doesn't really solve my problem. I'm trying to limit to activity vocabulary that the workflow author has access to. If they can specify their own custom type as the root activity of their workflow, they can also add whatever execution logic they want, which is what I'm trying to avoid.
There is an example in the Advanced Authoring chapter of Essential WF where they describe building a root activity class that generate top level properties as part of the workflow compilation process. The root activity (in the example, it's called "SequenceWithVars") where you can specify the top level parameters in the XAML. Here's an example from the book:
The SequenceWithVars type includes a custom ActivityCodeGenerator that loops thru the VariableDecls collection and adds a top-level property (via CodeDOM) for each VariableDecl instance. You end up with a workflow class that looks like this:
In order to use the XAML workflow with the SequenceWithVars activity, you do need to compile the XAML first using the WorkflowCompiler class. WorkflowCompiler.Compile() returns a compiled type which can then be passed to CreateWorkflow. But adding the separate compile step is a small price to pay, in my opinion. This approach lets me limit the workflows to XAML only while still allowing for top level properties which are needed in many data binding scenarios.
Update: removed syntax highlighting because it looked bad in my news reader.
Later Update: Fixed syntax highlighting.