Rob,
I'm going to treat him as "loose equipment".
FED 1-3-7, Penalty.
I agree with those who suggest ruling the ball dead and awarding bases if, in the umpire's judgement, the errant throw would have entered DBT had it not hit the base coach.
I believe in FED he is considered bench personnel not authorized to be in LBT. FED 3-3-1a.
JM