Can you try and see if something like this works:

```jinja
{% set nginx_app = salt['pillar.get']("apps:" + appslug + ":nginx", {
        'port': nginx.port,
        'auth_basic_user_file': '/etc/nginx/htpasswd-'+appslug,
        'auth_basic': False,
        'auth_basic_users': {},
        'redirects': [],
        'http_access_rules': [],
        'external_url': 'http://www.example.com',
        'enforce_www': False,
        'enforce_no_www': False,
        'is_external': False,
        'client_max_body_size': None,
    }, merge=True ) %}
```

Yes, merge should be a bool, not the value to merge: [pillar.get docs](http://docs.saltstack.com/en/latest/ref/modules/all/salt.modules.pillar.html#salt.modules.pillar.get)