None of it works. See this thread for more information. There is plenty posts about people trying to 'force' loperamide across the BBB. None of it works well enough to be worth the effort, though.
Tonic water contains quinine. A very small amount, at that. Quinidine (not quinine) is a drug usually used in malaria treatment and in high doses (600mg) has been shown to inhibit p-glycoprotein enough to let loperamide cross the BBB in significant amounts to become centrally active. This is dangerous of course, as quinidine has plenty of side effects of its own, especially at those doses. Besides you really can't get something like that unless of course you have malaria.
Don't try to get high on loperamide, it's not exactly the most recreational for one thing.
The effects are unpleasant at best at high doses. Although I will admit there are effects, people do abuse it and obviously have been addicted to it, but you seriously shouldn't go messing with your P-GP and blood-brain barrier.