this would allow custom formatting for example to
insert intra-tracker links into messages ('@bug36'), as
well as other
jazz.

The easiest way to achieve this would be to provide a
default
(no-op) parser/formatter that all messages are passed
through
when the page is rendered, and let instance admins provide
their own subclasses to provide the desired formatting
(in the instance specific 'interfaces.py' module...).