How to customize Finder's Share menu?
I haven't tried it but I would suggestion following : 


Copy your automator service in /System/Library/PrivateFrameworks/ShareKit.framework/Versions/A/PlugIns/
rename it Servicename.sharingservice
Now edit /System/Library/PrivateFrameworks/ShareKit.framework/Versions/A/Resources/SHKServicesOrder.plist by adding your service. (CFBundleIdentifier in Info.plist + function name)