Eloqua suggests that you have separate field merges for forms and emails. One that has a default value and one that does not.
That being said, I believe if you are planning to use the Progressive Profiling cloud app, you can set a default value for that particular form with the app. You would have to configure a new instance of the app for each of your forms, but you could set default values for them each individually if you wanted to, or not at all.