I actually did a variation of this. I created a text box in a non-displayed section that captured the UserName. Then based on that UserName I created rules to run and stopped other rules from running. I used action rules to say if the UserName contains xxxx then populate another text field with their email address. I then created a variable and set the value as the text field containing the email address. I then created an approval process that uses the variable as one of the approvers. And it works.
Of course the answer came to me after work.