Today, Sarah Cole found a web article that exams past Christmas meals; one of them being powdered geese. You might remember at least one episode where Uncle Fletcher spoke of powdered rabbit. I had examined in (what I thought was) some detail and came up with the conclusion that he was talking about some sort of glue... it made no sense then and now with Sarah's find, makes less sense.
Powdered seems to be another word for salt, probably being packed in salt for preservation.
Why and how Paul Rhymer knew about this is beyond our knowledge.