One of the big differences between archiving and backup is that in archiving I want to keep this exact version intact, if it changes on me it's an error while a backup takes a copy of whatever is now - maybe I wanted to edit that file. Unlike backups I think it's not about versioning, it's about maintaining one logical instance of the archive across different physical copies. Here's what I'm thinking, you create a system with three folders:
The archive acts like a CD/DVD/BluRay and is read-only. So far, nothing but a really awkward way to create a WORM(-ish) drive, but the real point comes next in distribution and synchronization.
When you put a file in "to_archive" a job will pick it up and wrap it in AES (with AES-NI the cost of on-the-fly encryption/decryption is very slim) and create a torrent-like file for it and move it to archived. If you want to delete it from the archive, you drag the file to the "to_trash" folder or maybe you put some kind of lock/freeze/undo timer on that function. Files that are in "archived" are sync'ed to other computers - still encrypted - which means you can shop around for storage/bandwidth, maybe you got multiple locations yourself (home/cabin), maybe swap backup with friends or family or you can buy it on the open market and they'll all mingle and share data because it's based on basic torrents.
They can all do basic limits on size/bandwidth so you can have pricing plans and caps, you can have one-way "leeches" that download and archive it on tape that can physically deliver it to you. If you build it fairly smart you can also have local, offline backups and if you restore them it'll pick up that 95% is the same as last week and sync up the rest. Basically a "Redundant Array of Inexpensive Archive Locations." It will leak a little bit of metadata as to size and number of files, but not file or directory names and you can probably muddle that metadata up with padding and dummy files if you want.
Of course you can choose to have the AES key on several computers so you can access your media from any of them. And as a free bonus a device that has the AES key like say your cell phone can use this as an online library, it doesn't have to auto-sync everything. With many locations = many peers it won't matter if one is down and you aggregate up the bandwidth, just like in any other torrent swarm. Through the seed/peer numbers you can at any time watch the state of your backup in progress as you add files. If your computer goes to shit, tell it the archive key and it'll hook up and start syncing. Just like a torrent client you can set priorities on what to download first.
It's not for all your data, but I think a lot of common user data is that way. Those RAW photos or video or audio you took? Archive them, "single" everlasting master copy. It doesn't replace backup of say documents you're working on or source code you're developing but it complements it.