I believe that a tagging system may be a bit complicated. I think a sign in sheet would work fine. One person man's the chrono station with a sheet and marks what player and gun is checked as OK. Before gameplay, an organizer can do a quick count of the sheet vs. the people. If there are 20 people and 20 sign ins, game on. A mismatch would mean that there has to be a roll call. I think we are all honest people here, and as I said before it is just as much a service to the guy with the gun as the other players. I don't think anyone is looking to break the rules to gain an advantage and certainly not purposely putting other people's safety at risk by swapping out guns, though people that went to RO know that kind of stuff happens. If you have a backup or a second weapon you want to use, chrono that up front, or else you'll have to wait for the chrono guy during inter-game time.
Whatever system would be used, I think the greater challenge is how to handle when someone has a hot gun. It would really suck to have your only gun come out hot. However, it is the owner's responsibility to ensure he has a weapon to spec before going to a game, just as much as it is their responsibility to have enough water or sunscreen. As a further complication, we all know each other pretty well, so it may be hard for the chrono guy to tell a buddy they can't use their gun. I guess the best way to look at it as a gun owner would be to think of it as your gun not working properly, instead of not firing well, it fires too well, best to take it home and work on it than chance hurting someone.
As far as leniency, there has to be a cutoff point. The important thing is that you stick to that point. If it's 400, it's 400. You could play the slip game until you get back to 430.