While you can't directly control/ configure the Expression section, you can manipulate it in certain ways - you would first need hit the 'Configure Validator' setting and add a Parameter.
For example, try adding a Parameter called 'test' with a baseType of 'STRING'. Now, in the expression field use something like 'result = test'. Bind a Textbox to the 'test' parameter, bind the output to a Checkbox, and add a Button with the clicked event bound to 'Evaluate' on the Validator widget. Now, when you view the Mashup, if you type in 'true' and click on the button, the checkbox should be enabled/ selected.
If you want to make the Expression section directly bindable, you would need to create a custom widget for that.