There has been much discussion on this forum about backup. I think you approach back up like you would for any life connection. The safeguard and probably most mechanical devices, warn against pulling something into your mechanical device that could cause catastrophic failure. Let’s say you tied a knot on your lose tag end of the rope, near the safeguard in case it slips. If the Safeguard actually slipped, the knot could be pulled into the Safeguard and cause failure of the Safeguard. I don’t think you want that, but each must decide what backups they feel are necessary for safety.
I personally either add an auto block and carabiner to my lose end of the rope clipped to my lineman loop as a backup. I don’t think it could get pulled into the safeguard clipped to me. Or I use my additional lightweight tether I carry, and simply put it on the tree below my primary tether. Basically a complete backup tether and connection. I connect this backup connection to a very short secondary bridge. This bridge is made of two climbing slings girth hitched to each lineman’s loop. So it hangs open, one on each side and I pull them together and clip them onto the backup tether carabiner. The climbing rated slings are light weight, strong and serve backup duties. I can also use the slings to clip things to during the climb, like my bow rope.
I like the auto block because it provides backup for the safeguard at height and during the rappel. You have to keep one hand on the auto block and the other on the lever during the rappel.
However, I like the backup tether and bridge at height because it contains redundancy for all life connection points; rope, carabiner, device or prusik, and bridge. But you don’t have backup during the rappel.
I find myself sometimes doing both. I prefer to keep things as simple and clean as possible. For me, that makes me safer. It‘s easy to get too much going on. Each of us have to decide on what makes us feel safe and what effectively makes us safe. And it’s good to hear what others do, even if it makes you think, or critically evaluate what you are doing.