This link doesn't offer any scientific support for the use of charcoal remedies specifically for snakebites (scroll ~3/4 down for How Can Charcoal Be Used On Snakebites?) but does describe a variety of acknowledged uses. Seems like an often overlooked item that would be worthwhile to include in a first aid kit.

This might be the book Bryan was referring to (endorsed by the co-founder of the Uchee Pines Institute, who's also a medical examiner in GA)