If you are copying a bootable image, it should work fine. But if you are trying to make a custom boot image it may be a bit difficult. Obviously 9 and 10.6 boot in very different ways... OS 9 needs a valid System Folder, and it needs to be blessed or touched or whatever. An X bootable CD will have a (usually hidden) setup of a boot file, mach_kernel, and stripped down folder structure with /bin, /usr, /.Trashes, etc. I am on my Windoze box now but I will need to see if OS 9 boot discs might need to be straight HFS format, as opposed to HFS+. Out of the box 10.6 will not support creating and writing to an HFS image, which could be the problem. I have heard that there are 3rd party extensions which allow it though.
Snow Leopard is great IMO but with the loss of writing HFS and AppleTalk, it is the first MacOS which really does not play nice with legacy systems.