So... I went looking for an alternative way. Setting up Windows 7 (+ Security Essentials + Inventor 2011) is not such a pleasure I would voluntarily look for it (again). So I want to "image" the bootcamp partition somehow.
Ideally, the image would only carry the bytes actually used in the NTFS volume and not be a 1:1 copy of the raw device. So Linux-like bitwise device copy is ruled out.
I was surprised not to find a ready-made solution for this in the otherwise splendid Coriolis iPartition 3 software. But their customer support gave the following advice:
Hi Asko, Thank you for you comments :)
Actually, there is a simple way to do what I believe you wish to do using iPartition and Disk Utility. You can use Disk Utility to create a "sparse disk image" large enough to accommodate your windows partition.
Then, in iPartition, delete any partitions which may have been created in your new disk image then navigate to the windows partition that you wish to back up. Select this partition and chose "Clone..." from the Partition drop-down menu. Set the target disk to your new disk image and hit Go.
You can now unmount this disk image and you will see that the image (.sparseimage) file size is only the size of the files that Windows was using. This is because iPartition recognizes the free space in your partition when performing the clone operation.
We hope that you find this solution useful. If you need any further assistance or have any other issues then please don't hesitate to get in touch.
Kind regards, (person's name removed)
I'm doing that right now, and it seems to work (*knock knock*). I do wish they would build this procedure (or similar) and restoration of Boot camp partition from the backup such created, right into iPartition.