register = template.Library()

@register.tag
def test_object(parser, token):
    try:
        tag, value = token.split_contents()
    except ValueError:
        raise template.TemplateSyntaxError('%r tag requires a single argument' % token.contents[0])
    return TestObject(value)

class TestObject(template.Node):
    def __init__(self, value):
        self.value = value
        
    def render(self, context):
        obj = utils.get_object(context['request'])
        context[self.value] = obj
        return ''

{% load my_tag %}
{% test_object BigObject %}