rdiff-backup user, part time programmer. I would love to see some more work on rdiff-backup, love to get some bugs fixed and see some performance increase, not going to comment on rewrite or fix the current code base - I haven't really looked at the code.
But I would say on encryption and de duplication - why not leave that to the filesystem - stay focused on what rdiff-backup does best - differential backups, you can get de duplication, compression and encryption file systems why not leave it to them to do that (well atleast for linux and any os that accepts fuse filesystem).