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][1]

> Enzymatic browning in avocado puree and
guacamole was evaluated by reflectance
measurements for several varieties of av
ocado with varying amounts of chemical
additives. Paste from Booth
8 had a greater tendency to br
own than that from Lula.
Browning at 75°F was effectively retarded wit
hout flavor change by 30 mg percent of
sodium bisulfite or 200 mg percent ascorbic
acid. Lemon juice or citric and hydrochloric
acid adjustment of the puree from pH 6.6
to 5.1 accelerated br
owning of Booth 8.
Combination of sodium bisulfite and ascorbic
acid were particularly effective in
preventing browning in all avocado pre
parations under all handl
ing conditions -
refrigerated, frozen and thawed, or
freeze-dried and reconstituted. 


(Booth 8 and lula are different varieties of avocado. You probably used [Hass][2].)

Looks like it's an enzymatic reaction that produces the browning: 


> The efficacy of sulfur dioxide in retarding
browning is thought to be due to the formation
of addition products with the
polyphenolase enzyme, thus in
terfering with the catalytic
activity. 

So a little bisulfite might be called for if you *must* prep the stuff long in advance.


  [1]: http://www.avocadosource.com/Journals/FSHSP/FSHSP_VOL_81_PG_230-235_1968.pdf
  [2]: http://en.wikipedia.org/wiki/Hass_avocado