Stormerne: Adding a new restriction to the game, when existing games were played without it, will mean that some of those games could not be replayed under the current rules. This would be an issue especially when new players are learning to play by example, by studying old games. And since my last message I've wondered: what would be done about games in progress under the existing rules? Unless tablut were to be "closed" during the course of those games, the two rules would have to run side by side anyway. I like txaggie's suggestion of having a new variant, as this site does for so many other games.
My preference for raichi is this: if we want to recreate an authentic "raichi", to prevent unsatisfying victories in which the opponent has not noticed the position, I think a more authentic way would be to have the system put the word on the screen somewhere, in the turn of the player who is threatened. The obligation for the threatened player to do something about it is not mentioned in Linnaeus, but having "raichi" in big letters above the playing board when you start your turn (and "check" in chess for that matter) would feel more authentic to me.