If you're assuming that every tenant's block device is storing a filesystem, then you're not providing your tenant a block device, you're providing your tenant a filesystem. And if you're providing them a filesystem, then you should use something like LVM for dynamic partitioning.

The point of NVMe namespaces is to partition at the block device layer. To turn one physical block device into multiple logical block devices, each with their own queues, LBA space, etc. It's for when your tenants are interacting with the block device directly. That's not a hack, that's intended functionality.