Add option in checkout to "pay using my stored credit card" which then uses the eWay Token Payment API

Thinking out loud:

For this to work we need to store the eway ManagedCustomerID against the drupal user.

So we could store in a new database table:
uid, ManagedCustomerID, cc last 4 digits, cc expiry

Then at checkout you let the user select from a list of their previously used cards.

We will probably need a new payment type as we don't need to capture the credit card details as we usually would with a credit card payment.