Not sure how "DTSTAMP" can't "vary across recurrences". The "DTSTAMP" property is afixed to each iCalendar object as it is created, prior to sending (e.g., in an iTIP/iMIP message).
If I send out a "REQUEST" for a Monday meeting, it has one DTSTAMP on that iMIP message. If I revise it by sending out an "ADD" for additional occurrences (e.g., Wednesday and Friday of the same week), then it will have a different DTSTAMP.
Hence, the two messages have different "DTSTAMP" values specified. So what did you mean?
-- Frank