Ok, I've managed so solve the issue by booting in normal mode (luckily the system was still functional), installing e2fsprogs from source, booting into rescue mode, mounting the FS, copying "ext2resize" from the mounted filesystem to the rescue filesystem, umounting again the filesystem, running the ext2resize tool, fsck'ing the disk and then shrinking the logical volume.
Thanks anyway; you can mark this thread as solved if anyone cares.