How can I customize the Guest user Dock in Yosemite?
Right after posting this I thought of another approach. It's not perfect but it's the best I've come up with, and it mostly works.

After copying my desired configuration to /System/Library/User\ Template/English.lproj/Library/Preferences/com.apple.Dock.plist, I ran chflags uchg /System/Library/User\ Template/English.lproj/Library/Preferences/. (I'm not sure why the flag needs to be set on the Preferences folder, rather than on the plist file itself, but that's the only way it works.) On logging into the Guest account, my customized Dock was intact! The only problem is that the user creation script still adds icons for Maps and iBooks, but that's not the end of the world. 

If anybody else has any other ideas, I'd love to hear them. I want to believe there's a better solution, but at least I've got something that works now.