This is an indirect answer -- not about the XSL export process. That info is in the
comment. Below is a MySql query which exposed the order comments for me (which I renamed that column to
SELECT sfo.entity_id, increment_id, customer_id, grand_total, sfo.shipping_amount, tax_amount, discount_amount, subtotal
, customer_email AS 'email', customer_firstname AS 'firstname', customer_lastname AS 'lastname'
, sfop.method, cc_type, cc_last4, sfop.additional_information as 'pmt_details', sfic.comment AS 'order_note'
, total_item_count AS 'order_lines_count', total_qty_ordered, sfo.created_at, sfo.updated_at
, remote_ip, shipping_description, customer_group_id, billing_address_id, shipping_address_id
, tm_field1 AS 'year', tm_field2 AS 'make', tm_field3 AS 'model', coupon_code
, tax_percent, customer_is_guest, customer_note
FROM sales_flat_order AS `sfo`
LEFT JOIN sales_flat_order_payment AS `sfop` ON sfop.parent_id = sfo.entity_id
LEFT JOIN sales_flat_invoice_comment AS `sfic` ON sfic.parent_id = sfo.entity_id
WHERE sfo.status = 'complete'