There isn’t a way to do this directly. There are a couple of options that I can think of but they have problems.
Since you are populating the fields in the page dynamically from the options you could add another radio button separate from your repeater to somehow indicate the default value. You could populate the values of this radio field dynamically based on the values entered into the repeater. The problem with this one is that the new radio field won’t populate with anything until you save the options… unless you build some javascript/ajax functions to do it… which I wouldn’t be able to help you with.
The other choice is to put a checkbox in the repeater for “Use as default”. The problem with this one is that there would be nothing to stop someone from checking multiple rows as defaults.
