[concept_8]

I'd say if it's a large enough game tell the other team leaders and show them and just get them to help advertise the surprise to the other ppl on the teams and you do the same, so there's only a few ppl from the opposing teams who know and everyone else just knows that there could be surprises for the to look out for.

That way team leaders know about it and can help judge if it's safe enough or a good idea but everyone else including ppl on your team have a surprise during the game and it's still fun for everyone.

Either that or screw the other teams, and like you said if it tosses bb's safely, and as long as they call out why should it be that big of a deal if they don't know about it or see it comming.
It's not like there isn't plenty of grenades and mines or claymores or even launchers on the market as it is.
Why aside from being surprised by one during a game should it be a problem when anyone can have one with enough cash or enginuity.