If you have cast sheet, it will have to be hammered or rolled; and it will have to be annealed in between each roll or after a bit of hammering. Do you have the a rolling mill and a way to anneal the sheet?
I can cast brass and silver wire and sheet, but I wouldn't want to do it as long as I can buy sheet. I can't imagine that all the early gunsmiths cast sheet. If you are going cast sheet, why not cast the part you need, directly? I have seen original longriflles with cast patchboxes, toe plates, sideplates, and nose pieces. Thimbles were usually made of very thin sheet, thinner than I would normally use; making me think that they used what they could easily acquire in a usable form.
I know that doesn't answer your question, but I thought the questions were in order. How big did you need? I have a little jewelers mould for casting sheet and wire to be rolled or drawn. The little ingot for sheet is about 2" sq and about 1/8" thick. I wouldn't mind casting you an ingot and sending it to you if you could use it. PM me if you want it.