On one end you have the body wood and on the other end you have the trem block which is metal, and you are merely trying to join them. So I would think that if it were made out of either of the two materials, it would create a near identical junction, since the body wood is the less dense of the two. Its sort of a "weakest link" in the density category. So as long as you used material that was as hard as the body wood, you should see the same net result. Adding weight to the guitar, however, with a brass block will alter the whole deal because it changes the overall mass of the body as a whole. I think it's far more important that you have a tight, or "high pressure" fit if you're going to benefit tonally. In other words if you have the springs adjusted so there's just barely contact to the block, it will still protect you during string breakage but not string bending/detuning. And it certainly will minimize any transmission of vibration to the body. The material you use would be irrelevant anyway. If you have a nice tight torque on the springs, or you've blocked it under pressure from both sides to make it like a hardtail, then you'll hear the improvement.