Breath patterns in texts What is the canonical reference if I wanted to read about breath patterns and their mapping to specific mind states? 

A while back I came across an online text which contained some very hard-to-remember tables of breath sequences. It was, if I remember correctly, a Visuddhimagga style Theravada text. It listed breath sequences such as a 2s-4l-2s breath sequence (s-short, l-long) followed by 2s-2l-2s etc. and mapped them to specific stages on the way to Jhanas and nanas.

It also had explanations about the kind of person who would get a certain sequence - for example a person who's upheld sila would get so-and-so breath pattern as opposed to a person strong in samadhi etc.

Does this sound familiar to anyone?

I'm open to such texts from any tradition.
The breath patterns you are mentioning (2s-4l-2s breath sequence (s-short, l-long) followed by 2s-2l-2s) is generally found in Yoga / Pranayama than in Theravada Buddhism.

Nevertheless, Anapana Sutta does mention about the long and short breaths. The breath is considered a bodily fabrication and it is tied to the body. (If you stop breathing then you are dead.) In meditation you have to calm your fabrication, out off which the bodily fabrication generally cease before mental fabrications. As you get concentrated your bodily fabrications also slow down hence shorter breaths. When you come out of meditation your breaths become longer as you start creating fabrication. If you start thinking and pondering (verbal fabrications) again your breath becomes longer as these fabrications influence bodily fabrications.

Kamabhu Sutta, Culavedalla Sutta &amp; Rahogata Sutta covers more on the 3 fabrications out of which bodily fabrication is tied with the breath and breathing process.