I'd love to see and hear how your bench turns out, I've been tempted to try one myself.
I think it would work as long as the stones are not greasy. You could ask at the stained glass and mosaic forum over on the home side of garden web. Sometimes they stick stained glass or what ever in a pattern to clear sticky shelf paper and put it in the bottom of stepping stone mold. That way the pattern doesn't move when you are shaking out the bubbles or what ever. (Just remember what you are looking at is the underside, its backwards) Here is a link to one project. Its called the Indirect method. There are detailed instructions somewhere, but unfortunately I didnt' save them.
good luck
kathy

Here is a link that might be useful: Indirect method of making a designon concrete

The indirect method is the way I would like to go. I'll let you know how it goes! BTW, I'll be using Quikrete's buff colorant, which should give it a more natural look. Here's the link for the indirect method:
http://www.inlandcraft.com/howto/mosaic/mosaic2.htm
Bill