base.html

{% load sniplates %}
{% load_widgets core="widgets.html" _soft=True %}

{% block content %}
    {% include "included_widget.html" %}
{% endblock %}

included_widget.html

{% load sniplates %}
{# 1.7+ fails without load_widgets here #}
{# {% load_widgets core="widgets.html" _soft=True %} #}
{% widget "core:alert" text="No boom today.  Boom tomorrow." %}

page1.html

{% extends "base.html" %}
{% load sniplates %}

{% block content %}
    {{ block.super }}
    {% widget "core:alert" text="There's always a boom tomorrow." %}
{% endblock %}

urls.py

from django.conf.urls import patterns, include, url
from django.shortcuts import render

urlpatterns = patterns('',
    url(r'^$', render, {'template_name': 'page1.html'}),
)

widgets.html

{% block alert %}
    <div class="alert">{{ text }}</div>
{% endblock alert %}