I actually set about finding out what metallic taste was years ago. It had nothing to do with LSD, rather, I was intrigued with the question of why licking a battery should taste metallic. It seemed like there was something special about it; it seemed to arc over numerous sensory modalities. I couldn't discern where the feeling of an electrical shock on my tongue ended and when the taste began.
I found out "metallic" taste is still of interest to taste perception researchers. Participants in experiments report a metallic taste when holding an iron solution in their mouths, but not when their nostrils are blocked. Conversely, they report a metallic taste when an electrical current is applied to their tongues or inner lips whether their nostrils are blocked or not, suggesting that without electricity, metallic taste only arises in conjunction with metallic smell. I thought, "well what about the metallic taste of pennies?" I discovered that, together, the zinc core and copper plating of pennies produces an electrical current, and thus metallic taste.
In the late 90s, a few patients receiving brain surgery agreed to have electrodes plunged into their thalamic taste relays. When an electrical current was run directly to this area of their brains, the patients reported a metallic taste and an acidy smell. The area is a nexus between different sensations. LSD is conducive to cross-signaling in the brain, so I wouldn't be surprised if its synesthetic tendencies, combined with possible increased excitation caused by it in this relay, was behind the metallic taste hallucinations.