I'd love to hear his theory on why mercury would have this magical property. What could it possibly be doing that would make an animal "explode". The bullet would be heavier and going slower. It wouldn't be any different than loading a heavier bullet, except it would be a lot more work and a lot more dangerous than just using a heavier bullet.
As for a discussion of custom/modified rounds, I guess you'd have to define that a little more narrowly. Almost all handloaded ammo would be considered "custom", unless it's an exact duplicate of factory ammo, and there are all sorts of ways that a round might be "modified". Any of the many Ackley Improved versions of standard cartridges could be considered "modified", for example.
A few guys on this forum have rifles chambered in unique, on of a kind cartridges, even. Doesn't get more "custom" than that.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.