gddrescue dosen't image the file in order - it goes back and retries, and fills in the blanks, and I suspect this is why you can't effectively pipe it into something else. The man page goes into that in detail
GNU ddrescue manages efficiently the status of the rescue in progress
and tries to rescue the good parts first, scheduling reads inside bad
(or slow) areas for later. This maximizes the amount of data that can
be finally recovered from a failing drive.
The standard dd utility can be used to save data from a failing drive,
but it reads the data sequentially, which may wear out the drive
without rescuing anything if the errors are at the beginning of the
Other programs switch to small size reads when they find errors, but
they still read the data sequentially. This is a bad idea because it
means spending more time at error areas, damaging the surface, the
heads and the drive mechanics, instead of getting out of them as fast
as possible. This behavior reduces the chances of rescuing the
remaining good data.
Its by design, and the solution, unfortunately, is to get a bigger drive. For the -S argument to work, I believe the used space on the source drive must be smaller than the destination drive.