The Linux kernel has well-defined internal interfaces for character streams, block devices, block-erase devices (mtd), and extent devices (LVM).

Has it been considered to have an official (but not exposed to userspace) "btree device" interface?

The idea being that you could write composable wrappers for btree devices the way you can write composable wrappers for block devices (dmsetup, etc). And have a common interface for these kinds of devices -- the kernel has at least two large and well-developed btree-on-a-block-device implementations (bcache/bcachefs and btrfs). Both of these implementations have been criticized as being quite monolithic and not as unixy ("many small sharp tools") as LVM/dmsetup are.