Mounting the iSCSI to the docker host and then sharing the volumes to docker containers might not be so crazy in terms of hard linking. Though I'll admit I'm not too familiar with iSCSI and I'm assuming it's a block device and you create a file system (e.g. ext4) on top, where the file system would support hard links.
I hadn't heard of Trash Guides before I first set mine up and I did it all poorly (used 2x the storage than I needed) and realised too late, so hopefully this helps!