Nice experiment. Oddly enough food scientists in Florida have looked into the same problem, and achieved results similar to yours:
THE RETARDATION OF ENZYMATIC BROWNING IN AVOCADO PUREE AND GUACAMOLE
Enzymatic browning in avocado puree and guacamole guacamole was evaluated by reflectance measurements measurements for several varieties of av ocadoavocado with varying amounts of chemical additives additives. Paste from Booth 8 8 had a greater tendency to br ownbrown than that from Lula. Browning Browning at 75°F was effectively retarded wit houtwithout flavor change by 30 mg percent of sodium sodium bisulfite or 200 mg percent ascorbic acid acid. Lemon juice or citric and hydrochloric acid acid adjustment of the puree from pH 6.6 to to 5.1 accelerated br owningbrowning of Booth 8. Combination Combination of sodium bisulfite and ascorbic acid acid were particularly effective in preventing preventing browning in all avocado pre parationspreparations under all handl inghandling conditions - refrigerated refrigerated, frozen and thawed, or freeze freeze-dried and reconstituted.
(Booth 8 and lula are different varieties of avocado. You probably used Hass.)
Looks like it's an enzymatic reaction that produces the browning:
The efficacy of sulfur dioxide in retarding browning browning is thought to be due to the formation of of addition products with the polyphenolase polyphenolase enzyme, thus in terferinginterfering with the catalytic activity activity.
So a little bisulfite might be called for if you must prep the stuff long in advance.