> ZFS incremental snapshots and its send/receive are great, but my task is to create binary identical image of hard drive with bootloaders, partitions and so on.

If you use UEFI, bootloader is just a normal file on a normal (FAT) partition so no special magic is needed, but fair enough.

ZFS typically handles partitions its own way so no need to sync that.

But really: If you just want to keep to full drives in sync, at block-level so one can at any point be a stand-in replacement for the other, it sounds like what you want is RAID0. Isn't that what you want?

And with UEFI you can boot those RAID-volumes, even though they are soft-raid volumes.

Not saying your need doesn't exist, but that existing solutions which are more standardized and requires less administration seemingly already exists.