I tried looking at the text code, but quickly became lost; is there any documentation anywhere on how to get the baseline of a particular text object
Baseline is a property of font, not text object. I guess Pango may have something for this. If you find it, you can add a corresponding method to our font_instance class.
, or even just an overview of the text subsystem in general? Typography is a scary, complex field...
Bad luck. Our text system was recently entirely reimplemented by a brilliant programmer who unfortunately decided to stop coding soon after that. As a result, it is now very capable but largely unexplored and uncharted. It is not totally unpenetrable, however; recently I fixed several text bugs and got some superficial understanding of how it works, so you can try asking me specific questions.