How do you create an alias for iPhoto package folders in mountain lion?
This still is possible via a work around.
The right-click contextual menu does not contain an option Make Alias for the Originals/Masters directories, but it does for subdirectories. The option Make Alias always shows in the menubar under File → Make Alias but is greyed out for the Originals/Masters directories.
Workaround

Go to a subdirectory of Originals/Master in Finder.
Create your alias, ⌘+L.
Open the Info of the created alias folder, ⌘+I.
Click the button Select new Original
Press ⌘+⇧+G to enter a custom path.
Enter your path, e.g. /Users/my_name/Pictures/Aperture Library.aplibrary/Masters
Rename the Alias folder.