The compression is possible right now, said Blocktrades, witnesses just need to do it on their nodes. From what I understood, it's a service/option available for the entire hard disk but only compresses new files. It leaves uncompressed files like that. To compress a blockchain file, the witness would have to use a compressed image or rebuild it from scratch with compression turned on, from what I understood. Since I never ran a witness, I'm not super-familiarized with their terminology.