The Mac OS is extremely flexible in being able to boot from external HDDs with older versions of the OS installed on them.  It really is a simple matter to buy a USB HDD to dedicate to being used for Bryce, install the Snow Leopard OS onto it, then add in Bryce and other apps. To use Bryce, just reboot via the new HDD as startup disk. No need to trash your current OS used for everything else. I have several older versions of the Mac OS around for various legacy apps, all on ext HDDs.  If you no longer have Snow Leopard, there are people about with the OS disks for sale on sites such as eBay. 

If Apple were nice, they would have all their legacy OS software available as a free download, not sure if they do however.