Actually, I think a charge placed at/around the base like that would probably pop the hydrant body upward, stippping out the valve and creating a gyser. I don't think the cast iron pipe will take the stress and as the body goes ballistic, the guts get ripped out along with it.

Just cutting the body up higher with the det cord wouldn't likely open the valve, unless the blast pulled on the stem hard enough to strip the valve out as with a base blast.

Blast would probably know better what kinda charge it would take.