Hi Leif,
To be honest, this Google Group is probably a close analogue to the wp-hackers mailing list. By that, I mean if you're wanting any traction on a feature request or idea, this is probably not the place to get it. At the moment, the Google code site is probably best for that.
That said, _wpsc_ajax_purchase_log_send_tracking_email() is where the email is sent from, and it's just a wp_mail() call. wp_mail() has all the subject/content-type filters you'd need to conditionally set the content-type to text/html. Let me know if you need any more pointers on that.