Add Custom Fields to Send Email Activity
Tim, as far as I know, you can't currently add custom fields to the built-in Send an Email activity.

An alternative approach would be to use the scheduled reminder functionality.


create a new activity type for this email and create a scheduled reminder for this new activity type when status = completed (or some other suitable status) and time = '0 hours after activity date time'
You have access to all activity fields and custom fields as tokens in the scheduled reminder field so the simplest approach would be to include {activity.subject} as the email subject and put {activity.details} in the body.
Give it a go. I'd be interested to know if this approach works for you. In theory, it should work but I have not implemented it in practice.