RBD in its current version provides the deep cp command to clone an image including all its snapshots. I would really like if this feature was included into the GUI functions for disk/vm cloning/moving (maybe with an extra checkbox).
Hey, feature requests are best placed in our bugtracker https://bugzilla.proxmox.com. The forum makes it easy to lose track of them.
We are having plans to make extend the storage plugins to make it possible to defer certain actions to the plugin. So that we do not use the generic approach via Promox VE / Qemu but let the storage plugin do it by itself. This would fall into that category.