Yeah, I thought about using LaTeX. I've certainly used it a lot in the past, but became quite frustrated with the weird syntax and difficulty extending. Plus its a 1.2GiB download on OSX.
I may well generate latex as an output format as a first formatting step.
There is a smaller package (92M) which is often enough:
...and also your enemy. "Frienemy", I think the word is. hat said, it's a nice back-end for the lightweight markup system I'm using on my own book this month. The memoir document class is really good stuff once you figure out how to use it.
Why not make dsl on top of latex instead of inventing everything yourself. I dout that the ruby DSL he is writing is going to compile to a perfektly nice pdf.
As a first step it's likely to compile down to LaTeX.