There is one great problem with including the Weeping Angels in any d20 game (or rather, in any 3.x or later d20 game, at least, myself not having much experience with prior editions), and that is the problem of 'facing', or rather, its RAW absence.
The game mechanics explicitly neglect to track facing, and without that, either all of the Angels are always active, or none of them are ever active.
Now, you could go with the concentration check someone mentioned before, but that wouldn't be in line with what we see on the show, either. It doesn't take concentration to look at an Angel. They don't project a perception filter. They just turn to stone.