Duh!! For some reason, I just assumed there was a base plug but failed to think just how the lead would be poured in!
Gives me an idea!
How about a mold such as this with a good sized cylinrical sprue upon which donut shaped wads could be stacked?
That would insure that the bullet goes down the bore straight and if made of lubed felt, would seal the oval bore nicely.
Not sure if the wads would shed without upsetting the bullet path, but it could work. The bullet would be like a dart.