Display custom models with Interactive Markers

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I am writing an RVIZ panel for performing user guided model registration in a point cloud scene.

For the purposes of performing user guided ICP, I would like to display a custom model in the centre of an interactive marker.

However, I'm unable to find documentation on how to do so. I've looked through interactive markers, but nothing suggests they support this functionality. What's the best way to achieve a custom model in an interactive marker?