Non-blocking visualization with transparent meshes (and new visualizer)?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I have an experiment scene with estimated pose of objects represented as transparent copies of the object mesh. It seems like the only way to get transparency (I'd prefer a simple alpha, but only PBR transparency is supported?) is through the new visualizer with `o3d.visualization.draw` as demonstrated here: https://github.com/isl-org/Open3D/issues/4060

However, I'd like for my scene to be non-blocking and for me to update the geometries as they change during the experiment. With the legacy visualizer, I am currently handling that with `add_geometry` and `update_geometry`. How do I accomplish something similar with the new visualizer? I see that there is a `non_blocking_and_return_uid` option in the `draw` API, but I don't see how I can update the geometries.

I have also read the API reference for the new `O3DVisualizer` here: http://www.open3d.org/docs/latest/python_api/open3d.visualization.O3DVisualizer.html
but it seems like `update_geometry` currently only works for tensor point clouds. I need it for meshes.