Is it possible to retrieve the email address of the workflow initiator from an SDK script? I thought maybe there would be something to do with the UserInfo class, but there's nothing to do with email within that class.
Depending on how your user records are set-up, you may be able to get it with standard Workflow activities.
Try the "Find User" activity, with the option to search for a user from a token. There are two standard tokens %(Initiator) and %(Initiator SID), which match to the "Find User" activity's Account Name and ID options respectively.