I think the answer from this related question is pertinent; currently, the best solution seems to be to use a custom template loader from the django-apptemplates package on PyPI, so you can just use pip to install it (e.g. pip install django-apptemplates).
The template loader allows you to extend a template in a specific app; for example, to extend the index page of the admin inteface, you would add
'apptemplates.Loader',
to your TEMPLATE_LOADERS list in settings.py, and use
{% extends "admin:admin/index.html" %}
in your templates.