If an unwritten rule indeed exists, it’s not being respected. And it can’t be enforced. The Seahawks could have avoided the issue by continueing to carry Bowie on the 90-man roster until the first roster cuts and then slid him to injured reserve. Instead of making a move with one of the other 89 players, the Seahawks took a calculated risk on Bowie, and it backfired.
Didn't take as long as I thought for the real story to come out, but it's just as I suspected.