I found an alternate way to do it that doesn’t require a callback function or a dictionary. Basically you break out the scene and the mesh as separate cells, so they aren’t internal to your ‘create’ cell. Here’s the example:
https://beta.observablehq.com/@jbum/mesh-modification-alt-method
EDIT: I deleted this since Mike’s example is much cleaner. Also updated my sphere demo to use this style.