Is it safe to throw frozen meat in marinade?
Yes, it is perfectly safe (as long as you continue to thaw the meat in a safe manner, as in the refrigerator).

The marinade will not begin to have much effect until at least the outer layers of the meat are thawed, but it will not otherwise have any side effect.  It may get slightly better penetration due to the changes in the texture of the meat from ice crystals, but the effect should be negligible at best.

In fact, you can freeze the meat in its marinade if you so choose, and know what you are going to use it for when you thaw.  It will get the benefit of marination time until the outer layers are frozen, and then again during the thawing.