I cast a large number of balls from a small cast iron skillet on top of an electric stove, back in the 70's. Worked very well.
However and fair warning, make SURE you are wearing stout shoes when you cast balls over a stove. I didn't one time and the mold I had perched on the rim of the skillet slipped and fell into the pot. That sent lead flying and a glob landed on my sock covered foot. I did the "Native Hotfoot Dance" while trying to tear off the sock. :redface:
For your impression, I think you have the right idea to make lead strips from a wood mold, that would not be difficult to add to a small period pot. Since I'm sure you are not going to cast balls in front of tourists, may I suggest you leave a ball in the mold with the sprue still on it? That way you could show tourists what it looked like when you first take the ball out of the mold and maybe even show them how to cut the sprue.
Gus