Okay, I was wrong, wrong, wrong.
The AccountId has info you'll need, however. You'll need to create another field, outside of the person picker group and set a default value on it:
Then map that field instead. We would generally submit the user's ID number (which I'd hoped was in the AccountID field), but if we don't know what that ID is, we can use:
With UserName as the account id.
Now, if your users can select multiple people, we'd need to also use either a double eval or the DelimitedList command to aggregate the values. Does that make sense?
Also - this is for SharePoint 2010, not Office 365. We are still trying to work out the syntax for that....