I wouldn't recommend them since they're made of tin. Lye may react with it and you'd not only have wasted supplies but you'd ruin the mold. Although, they might work for candles.
You might be able to find silicone molds in these shapes if you look. I saw some on eBay made from plastic which had reviews about them working for cold process. Search for pillar loaf molds and the auctions should come up.