That's all right, I answered my own question. Power items do not affect the Shininess of a hatched Pokemon. However, by using the Everstone to set the Nature of a hatched Pokemon (assuming 100% chance in B2/W2), if that Pokemon has its Hidden Ability, you are reducing the odds of it hatching Shiny by one-fourth. Because Shiny odds are 8/65536 instead of 1/65536, what your Trainer ID/SID is has no real bearing on Shininess.
So, essentially, my odds of hatching a Shiny Poliwag with its Hidden Ability, and without the Masuda Method, are 1/8192 (base Shiny odds) * 60% (Hidden Ability chance) * 75% (offset for Hidden Ability from Everstone) = 1/8192 * 45% = 1/18204 (rounded to the nearest whole number).
Source:
http://bulbapedia.bulbagarden.net/wi...sonality_value