I think I've often seen CO GAs keeping track of seat numbers and entering boarding information in that way. If the GA saw your seat number and let you board, all (s)he'd have to do is go punch it in to the computer and you'd be all set. It's easy enough to remember a seat that (s)he wouldn't have to type it in immediately and still be able to do it accurately a few minutes later. It definitely has to be entered into the computer some way and I think this is the easiest "manual" way to do it. I believe I've seen it done this way for BPs that the machine refuses to read as well.