To be honest, this is a far-fetched idea. But maybe someone will do it. I would do it if I knew how in a flash, but as I said, I don't know how.
Think of it this way, a cover system doesn't need too much work (in blockland, at least?) The ideas I thought would just be a sitting position and a standing position on a vehicle spawn. The "cover" vehicle wouldn't be enterable until touching the vehicle brick (or something else through events perhaps) and once you entered it (as an example, say it's cover for a short wall) you were in a sitting position. By using the , and . keys to change "seats" you could stand up and sit down, thus using "cover"
Although you can basically do this by shifting, I think it would be a neat idea. People on BL nowadays seem to love servers with unique addons, so it could have interesting affects.
PLUS, if you really put the work into it, you could make many different cover blocks. I guess it's if someone wants to take it on as a challenge.