Just a thought....
I was wondering why you wanted to melt it in the first place too.
I suppose if you are borderline anal about accuracy and tuning (that can be a good thing) or maybe are just over thinking things, you might be afraid that the shot will end up kinda lop sided in the case and so figured pouring molten lead would be the best bet. Makes sense I suppose if you consider that the bb's could get all off on one side in the glue it might make the spinning arrow behave badly just like a crookedly installed point. Having just gotten into bullet casting recently, I can tell you that the idea of pouring molten lead cleanly into the bottom of a 357 casing is not likely to happen. So I'm like, "yea, melting it in the pan would be just the ticket." Then again, in the interest of simplicity, if a fella was to use the right size shot to get one perfect layer of bb's in the bottom of the case they wouldn't be able to move around at all really. I'm thinking one in the center with about 6 more around it. A drop of hot glue or epoxy on top would hold things in place nicely.
Then again, it's kinda fun to mess around melting lead. I'd give the hot pan or steel plate a try first, If that turned out to be a hassle, I'd try to find just the right size shot to put a single layer in the bottom and see what weight that gave me. Two layers should stack just as well as one if more weight was needed.
Let us know what you find out.
Just had another idea. I'm wondering if maybe a small steel washer of the right size would fit down in the case? Maybe a washer for a #8 or #10 screw. Gotta be pretty easy to go to Lowes and hit the hardware bins to see what fits.