Homemade Avocado Mayonnaise

In my continuing hunt for egg-free, soy-free mayonnaise, I ran across an article in Eating Well featuring Rick Bayless and avocados. The article mentioned that avocados are a natural emulsifier, so I ran to the computer and looked up recipes for avocado mayonnaise. Then I ran to the grocery store to pick out the perfect avocado - according to Bayless a perfectly ripe avocado is not green, but