Currently, with IP.Board 3.1.0, there are 4 hooks for topic view that allows us to add a new button beside a post, compute some data but we cannot add new field for the posts select query (like birthday).

I suggest to add a new hook for topic view that we could customize posts select query and add a new sql join.
For example, this new hook could be used to display easily and cleanly (without changing source code) age of members in topics.

But you already have all the fields you need for the birthday in topic view - $post['author']['bday_day'], $post['author']['bday_month'] and $post['author']['bday_year']

But you already have all the fields you need for the birthday in topic view - $post['author']['bday_day'], $post['author']['bday_month'] and $post['author']['bday_year']

Those fields are not in the $post['author'] array.

Oh, I see. I always thought that $post['author'] was pretty much a clone of an IPSMember instance.

I thought so too, I tried to make a hook that did just this (I had a similar mod for 2.x), but I found that only certain fields from the members table get pulled. A bit annoying.