However, it would be nice to come up with one that could do the job on a 100 yd target but not retain much energy at 500.
Conceptually, this is pretty easy. Make it low mass and low ballistic coefficient and use a low powder charge. That way it will not have a lot of kinetic energy to begin with and it will lose what it has quickly. But the trajectory will have a lot of curvature.
How this works out in practice would depend a lot on bore size and rifling or not. I'll bet with low powder charges, you could make projectiles from wood that would be adequate for 100 yards. Time for some experimenting, but not out the front door.