http://chriskief.com/2013/11/06/conditional-output-of-a-django-block/

captureas.py

from django import template


register = template.Library()

@register.tag(name='captureas')
def do_captureas(parser, token):

    try:
        tag_name, args = token.contents.split(None, 1)
    except ValueError:
        raise template.TemplateSyntaxError("'captureas' node requires a variable name.")

    nodelist = parser.parse(('endcaptureas',))
    parser.delete_first_token()

    return CaptureasNode(nodelist, args)


class CaptureasNode(template.Node):

    def __init__(self, nodelist, varname):
        self.nodelist = nodelist
        self.varname = varname

    def render(self, context):
        output = self.nodelist.render(context)
        context[self.varname] = output
        return ''

Awesome! Thanks.