Accessing a device through an offset

I thought I had found a way to do this, but now that I want to test the
procedure it was to be part of I can't find it.  :-(

Suppose you had a block device that had a foreign header of x blocks,
with the remainder of the device laid out as if it were a normal hard
disk, but with the MBR in physical sector x+1 rather than 1, etc.  Is
there some easy way to attach the physical drive to a driver that maps
blocks?  So you might do

  magic-setup-command -o some_number_of_blocks /dev/hdc /dev/ohd

and then you could fdisk /dev/ohd, mount /dev/ohd1, etc.

Workarounds to scrape the partitions out of the misplaced MBR and turn
them into offsets for use with a loop mount are known, but limited to
primary partitions.  Okay, I guess it could be extended to dig out
extended partition info as well, but that's not the direction I want to
go.  And then there are the LVM-managed partitions...

-- 
...that obsessive conviction, so common among authors and composers,
that all similarities between their works and any others which appear
later must inevitably be ascribed to plagiarism. -- 2nd Circuit, 1945
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