My theory on a mercury-filled hollowpoint is this:
Hollowpoints have a bugaboo: they're unreliable expanders. Regardless of the design, hollowpoints expand because of hydraulic action. Hydrashocks and Eldorado starfires use fancy internal configurations to distribute the hydraul;ic action to improve functional expansion.
Hydraulic principles work on the premise of "Liquids cannot compress". Hollowpoints driven into flesh rarely look like the perfect mushrooms from gelatin tests because flesh is not entirely a liquid, and makes for an inconsistent hydraulic medium. Additionally, hollowpoints can become clogged with cloth or other non-liquid materials on their way into a target, which prevents hydraulic influence on the bullet's interior construction, preventing expansion.
However, if one were to pre-fill a hollowpoint cavity with some sort of liquid, it would lessen the chance of clogging, and assist in the expansion of the bullet merely from forward impact.
Mercury's a lousy choice, for it's lead-disolving tendencies, as noted above. Wax would probably work ok.
Also, you can't fire paintballs from rifled barrels, as the liquid contents act as a fluid damper on the rotation, and quickly destabilize the ball, removing the accurrizing benefits. I would think that .filling a hollowpoint with something that remains a liquid would act to destabilize a bullet, considering the level of forces involved with the high RPM achieved in rifled bullets.
As for poisoning targets, as if shooting them isn't lethal enough, a drop or two of bore cleaner on any exposed lead would guarantee swift lead poisoning, as that changes the lead into a readily soluble form, as opposed to bullets, which are routinely left in people's bodies when they're in difficult to remove or non-threatening locations with no ill affects. Lead in mettalic form is reasonably inert. So is mercury, for that matter. Either, however, in vaporised form, or as a soluble compound, can be virulently toxic.