Django admin - how to get all registered models in templatetag?

You can access admin.site._registry dict of Model->ModelAdmin:

>>> ./manage.py shell

In [1]: from urls import * # load admin

In [2]: from django.contrib import admin

In [3]: admin.site._registry
Out[3]: 
{django.contrib.auth.models.Group: <django.contrib.auth.admin.GroupAdmin at 0x22629d0>,
 django.contrib.auth.models.User: <django.contrib.auth.admin.UserAdmin at 0x2262a10>,
 django.contrib.sites.models.Site: <django.contrib.sites.admin.SiteAdmin at 0x2262c90>,
 testapp.models.Up: <django.contrib.admin.options.ModelAdmin at 0x2269c10>,
 nashvegas.models.Migration: <nashvegas.admin.MigrationAdmin at 0x2262ad0>}

This is what the admin index view does:

@never_cache
def index(self, request, extra_context=None):
    """ 
    Displays the main admin index page, which lists all of the installed
    apps that have been registered in this site.
    """
    app_dict = {}
    user = request.user
    for model, model_admin in self._registry.items():
        # ...

Note that variables prefixed with an underscore are potentially subject to changes in future versions of django.