I just added a mechanism to find the `appleseed::studio::RenderTab` associated to a given tab. See for instance how it is used here:
https://github.com/appleseedhq/appleseed/blob/974b7e17ab1df5c7f5e29897a45877f8d9bcc7d6/src/appleseed.studio/mainwindow/mainwindow.cpp#L1275

You then simply need to add a `set_clear_frame_button_enabled(bool)` method to `appleseed::studio::RenderTab` and you're done without any `findChild()` hack.