We always have a requirement where we have to provide the user with a link of the document in their dialog workitem. You can easily accomplish by doing a Business Object->Business Object Binding from Workflow to task.
Once you have done you will get a link in the dialog workitem on which if you click will execute the default method of the business object. Generally all standard Business object use Display Method as the default method. If you need to change it i will request create a copy and change the default method. Please do not change the default method of the custom bo which you delegated from standard BO.