rsync -avz /dev/sda1 user@ip:/backup/ would attempt to copy the device node, not the disk content.
You can make an image of the partition as a remote file:
ssh -C user@ip:/backup/sda1.img </dev/sda1
This makes an image of the partition. It won't give you access to your files. In order to access your files, you need to mount the partition or the image: that's what mounting means.
If you only have one disk in a two-disk volume group, you won't be able to get your files from it, because you only have half a filesystem. Half a filesystem doesn't mean you have half the files, any more than having half a car doesn't mean you can travel on half the roads. If you have half a car, you can use it for parts, but you can't drive it. If you have half a filesystem, you may be able to use forensic techniques to recover some of the data, but it's hard work and individual files and directories are likely to be split over the two disks anyway.
If your volume group is split over two disks and one of the disk fails, your whole volume group is effectively lost.