rsync remote raw block device with --inplace

[Steve Newcomb]

It would be very nice to be able to rsync the raw data content of, e.g., a
non-mounted disk partition, particularly in combination with --inplace.

Our reality: several dual-boot machines running Windows during the day and
Linux at night, during backups.  Windows is very tedious and iffy to
re-reinstall without a raw disk image to start from.  Disks fail, and the
ensuing downtime must be minimized.

We're using dd for this.  Most of the nightly work is redundant and
wasteful of elapsed time and storage.  Storage is cheap, but it's not
*that* cheap.  Elapsed time is priceless.

Rsync refuses to back up raw devices, and even raw character devices, with
the message "skipping non-regular file" (I think the relevant message is in
generator.c).

In Linux, anyway, the "raw" command allows a block device to be bound as a
character device, and then even a "cat" command can read the raw data of
the block device.  So why does rsync refuse to copy such content, or why is
it a bad idea, or what rsync doctrine conflicts with it?  I agree there are
security concerns here, but rsync already disallows some of its functions
unless the super user is requesting them.
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