[all variants] Compressing a dd image in a way that allows direct mounting

Hi,

I have written a tool on my NAS, that automatically creates images from all disks in all computers on my LAN. It runs in a cron job and it does really sophisticated things, like clearing swap files and unused space, remounting the filesystems readonly before creating images, afm.

My problem: I want to be able to restore just a few files from these images, and because they can become VERY big it's quite likely that there is not enough space for uncompressing. So what I'm looking for is a solution to access the images without needing to uncompress.

So far I found two possible solutions on the inteet:
- Archivemount. But: Works only with tars, often stalls (maybe because of the filesize?), is not natively available on all target systems, and is extremely slow (general problem with seeking in regular archives)
- Squashfs. Perfect for how it works AFTER having created the images, But: File size is 1.5x of a regularly xz-packed archive, and mksquashfs doesn't seem to have an option to write it's output to stdout which makes it much more complicated to bring the output to the NAS: the script first has to establish a mountpoint to the NAS on the source computer - while it's filesystem is mounted Readonly! - and additionally it therefore has to pass credentials over the network.

Any ideas of how it could work better?

best regards
ako673de