Platform: iOS
TNS: 2.0.1

The value does work, but trace.js still complains:

<Page navigatedTo="navigatedTo">

  <Repeater items="{{ photos }}">
    <Repeater.itemTemplate>
      <Label text="{{ $parents['Page'].photoWidth }}" />
      <Image src="{{ $value }}" stretch="aspectFill" class="photo" width="{{ $parents['Page'].photoWidth }}" />
    </Repeater.itemTemplate>
  </Repeater>

</page>

Label says 375 (device width). It only complains at the width property of Image. One could assume that the value might be a string, but the error says it's null:

CONSOLE ERROR file:///app/tns_modules/trace/trace.js:160:30: Binding: Property: 'photoWidth' is invalid or does not exist. SourceProperty: '$parents['Page'].photoWidth'

CONSOLE ERROR file:///app/tns_modules/trace/trace.js:160:30: Binding: Binding error while setting property width of Image(157)@file:///app/main-page.xml:10:13;: Error: Invalid value null for property width