I am not sure about how 'seaplane' ratings are conducted - but probably not from airfields of any type, I would think.
The other activities to whic you refer do not add 'Ratings' to a licence, so are exmept from any requirement to use a licensed or government aerodrome.
The ANO change will list those items for which 'differences training' is mandatory; to add Class Ratings to an existing NPPL will require 'conversion training', not 'differences' training. But any SSEA training for a Class Rating must be conducted from a licensed or government aerodrome..........at the moment.