You quoted Foucault:
"The Archaeology of Knowledge,
Foucault wrote "...whenever, between objects, types of statement, concepts,
or thematic choices, one can define a regularity (an order, correlations,
positions and functionings, transformations), we will say, for the sake of
convenience, that we are dealing with a discursive formation."
This is what Foucault wrote. I'm more interested in what Foucault did. And
if you take a close look, Foucault studied mostly facts from what you could
call the Early Modern History (from the Renaissance to the Enlightment). The
fact is that you need to identify something in Bentham Panopticon that by
"an order, correlations, positions and functionings, transformations" is
still valid now. I sympathize with your desire that ethnographic
observations can unveil discursive formations, but a discursive formation is
not a Discourse. A Discourse is something much more perennial in Time. For
instance, Modernity had a Discourse and although Modernity as an existential
or historical condition was something that Hegel and his contemporaries
discovered, it started in the last days of the Middle Ages when some people
started to call consequently themselves "Moderns" in order to continue what
the Ancients have done.
First Disegno and then Design as well as Modern contributed to the Discourse
of Modernity because they were available when Modern was already something
detached from Antiquity.
It is very puzzling to me that Pevsner's "Academies of Art Past and Present"
is so often forgotten when it gives a very good account of the social role
of the Arts ancestral to Design.
Also it is interesting to remember what Victor Margolin selected to be
"Design Discourse" a few years ago.
PS: But I'm still more interested in situations in which Design as discourse
is not defendable.