Linux: Ability to access files of all datasets from one mount point

Describe the feature you would like to see added to OpenZFS

On Linux BTRFS allows accessing subvolume files from the root subvolume mount point. On ZFS, files on sub-root datasets can not be access via the root dataset mount point. To keep the scope as narrow as possible, I only want to be able to do this from the root dataset, not from arbitrary datasets to their children.

How will this feature improve OpenZFS?

It will allow offline deduplication poolwide.

Additional context

The motivating factor for this is offline deduplication across datasets. This is effectively disallowed, without patching the Linux kernel, because the FIDEDUPERANGE ioctl bails when using files from different mount points.