example-view.coffee

module.exports = class ExampleView extends View
  bindings:
    '[name=sub_field]':
      observe: 'field'
      onSet: (val) ->
        @setSubField 'sub_field', val
      onGet: (val) ->
        @getSubField 'sub_field', val
  
  setSubField: (key, val) ->
    field = @model.get 'field'
    field[key] = val
    field
  
  getSubField: (key, val) ->
    val[key] || ""

I believe in this case @ or this is bound to the class object, not the instance so it won't work.