The most serious effort at adding templating to Mailman is Ethan Fremen's SoC 2006 project.
The initial goals are described on Summer of Code.
The last-known status is described on Summer of Code summary.
As a first step, we'll work on using CSS in the existing 2.1 codebase.
The templating toolkit used is Genshi: valid-XML templates, with attributes and elements
as the markers for special behaviour and $var for variables. Worth revisiting this decision?