A lot of artisans use lead-free solid wire solder as a casting medium when they're doing just a couple small castings. Real casting pewter is about the same price, but can be difficult to find locally, and the solder is as close as the nearest hardware store.
One thing to keep in mind is that solder typically melts around 425 F, and depending on specific alloy, it can be significantly higher. Also, you may find it difficult to get the solder into a small void once it's melted, as it actually has incredibly high surface tension and you might not be able to get any capillary action to draw it into the void.
Another gotcha is to use solid core if you plan on melting it. Rosin core solder can be messy, and I'm sure acid core will make a mess too - though I have no direct experience with it.
You may be able to make it work by setting your soldering iron to the highest possible heat and working quickly and delicately to "smoosh" the solder into the hole - similar to the way wax burn-in is done.