gistfile1.py

class Location(Document):
  structure = {
    "ll" : validate_coordinates,
    "street" : ...
    ...
  }

class Venue(Document):
 structure = {
   "location" : Location.validate,
   "name" : unicode,
   ...
 }

 @property
 def location(self):
   # should be cached
   return Location(self['location'])

Or maybe use a custom type as the some embedded element for custom validation and the rest is just an dict structure...

class LongitudeLatitude(CustomType):
    mongo_type = list
    python_type = list
    init_type = [0,0]

    def to_bson(self, value):
        return value

    def to_python(self, value):
        return value

    def validate(self, value, path):
        if len(value) != 2 or not value[0] or not value[1]:
            raise SchemaTypeError('Bad ll')
        pass

class Venue(Document):
    structure = {
        "location" : {
            'street' : basestring,
            'll' : LongitudeLatitude(),
            ...
        },
        "name" : unicode,
        ...
    }

In fact that isn't even really necessary... it's just a list so we could override the validate() method on the Venue and implement the check there...