I've never seen one but they certainly made a packaged
replenisher for packaged D-76, which IS the buffered
variety. There was no replenisher given the 1929 paper
describing the buffered version of D-76 but this paper
predated the use of replenishing solutions so that isn't
surprizing. I suspect there must be a formlula somewhere
because buffered D-76 was widely used in the motion picture
industry in automatic processing machines for many years.
They require replenishment. It may also be possible to
replenish using the developer but the rate would have to be
determined by experiment. BTW, Kodak suggested adding 0.25
gram of Potassium bromide to the fresh developer to control
a slight fog it produced. In the 1929 paper this is one of
the variations and is reported to increase effective film
speed slightly. In a replenished system it would probably
result in a smoother starting curve compensating somewhat
for the build up of bromide as the developer is used. D-76
is nearly eighty years old now and is still the standard of
comparison.

  BTW, the creation of D-76 is attributed to John G.
Capstaff, of Kodak Research Laboratories, who is also
credited with devising practical home movies and the
reversal processing method for producing projection
positives directly from camera originals. D-76 was
originally intended for development of a then new
duplicating film for motion picture negatives. Its virtues
were low graininess, low and controlable contrast, and long
life. It was announced in a 1927 booklet from Kodak
describing the film. It was pretty rapidly adopted as the
standard developer for negatives, replacing a variety of
other developers, mostly variations of Pyro developers. It
became evident quickly that the activity of the new
developer was not stable with time; it tended to increase. A
research project was undertaken to find out why. The actual
chemical reactions were not discovered for decades but it
was found that the pH increased slowly with time. So, in
1929, a version with a borax/boric acid buffer was devised,
by Carlton and Crabtree of Kodak and published in an
extensive paper on the developer.
  It turns out that the pH rise is caused by a complex
reaction between the Hydroquinone and the Sulfite. This
produces a small amount of Sodium Hydroxide. It also
produces some very active Quinones but neither they, nor the
Hydroquinone, are active at the low pH of D-76 so they do
not contribute to the rise in activity. While the
Hydroquinone is nearly inactive as a developing agent in
D-76 it does act to preserve and regenerate the Metol, much
extending the capacity of the developer, and probably
affecting its reaction products in a way that changes
edge/border effects from what they would be with Metol only.
Interesting stuff, but enough already.