Because the classic template is most complete,
I suggest to add support for the local_replace.py
extension to it.

Here is a patch doing so.
(And I am also adding an example for a more advanced local_replace.py)

The patches add functionality to the extensions directory and
additionally makes this extension required by using it in the html
templates.

If it is required by the classic template, I'd rather see it integrated
into the core.

Additionally the configuration for the extension will happen in the
extension itself. Having a configuration mechanism similar to 
schema.py or interfaces.py (in the tracker root directory, in the case
of interfaces.py even an optional file) might be preferable.

One additional thought: Adding it as a default extension might be OK, if
the extension code and the configuration code can be separated and the
extension provides a sane default when running without further
configuration.