I'm not convinced that because all beasts (Maexxna, Auriaya's adds) have the same armor that all bosses everywhere have the same armor. That isn't to say that it isn't true necessarily - but I'm getting a lot of anecdotal data that would suggest some bosses (Mimiron tank) have very high armor, and some have lower.
Anecdotal evidence is just that. Until someone comes forth with some concrete evidence that the armor value for a given boss is different than 10,643, nothing is going to change in regards to how much armor a boss is assumed to have.
It shouldn't be too difficult to reverse-engineer any given mob's armor value, especially with a static damage attack such as Bloodthirst. Some of the posters here have done this extensively in their exploration of the Armor Penetration stat.
That being said, I believe it was assumed that bosses have a uniform armor value so that the DPS contribution of Armor Penetration would stay relatively predictable, up to and until you reduce armor to 0.
There was a fellow named Dayve who had an addon that would do it- as he did white damage, the addon would establish an upper and lower limit to the range and infer the armor value from that. He hasn't posted since Dec 08 though.