QRDA-I: Yes, with that caveat that CMS may further constrain fields in the header.
QRDA-III: There is a sample published with the HL7 standard
http://www.hl7.org/implement/standards/product_brief.cfm?product_id=286. There is a single XML format, although the exact content will vary depending on measure(s) being reported. For instance, some measures define a single Denominator population, whereas some define multiple Denominator populations. The QRDA-III will contain aggregate counts for all of the populations in the referenced measure or measures.