Mixed Classes in a DictProperty?

Wondering how can I mix different classes in a DictProperty. For example, in the example below I have 2 different classes that I would like to map to 'actions' DictProperty attribute.
Couldn't find any knob in the tests or documentation. Appreciate any comments about it.

# generic class
class Action(orm.JsonObject):
    pass

# action class 1
class EmailAction(Action):
    action = 'email'
    subject = orm.StringProperty()
    ...

# action class 2
class PagerAction(Action):
    action = 'pager'
    queues = orm.ListProperty(orm.StringProperty)
    ...

# using DictProperty to get it together
class Alert(orm.JsonObject):
    actions = orm.DictProperty(Action)
    ...