According to Michael Harlan's book on late
republican moneyers, Aelian describeds snakes being offered barley cakes in
his description of the ritual where maidens made an offering to
Juno Sospita at Lanuvium.
He also relates a story where a divine
snake in
Egypt is offered bowls of barley soaked in honey and milk set out on tables.
If we are talking about divine snakes, or a divinity feeding a
snake must the true diets of the
animals really be a factor. A mere mortal might not be able to get a
snake to feed on milk, but would a goddess necesarily have any problem so doing?
I doubt it is relevant but I recall that in the rituals of Bona Dea (also a "medical" goddess was linked to snakes) wine
had to be referred to as "milk".
Steve