How to make spotlight index files shared over AFP
I found (what seems to be) a good solution after messing around and observing another AFP share that is working correctly.  Turns out that the working folder has ACLs to allow spotlight indexing:

$ ls -led /Shared\ Items/Public/
drwxrwxr-x+ 19 root  admin  646 May  6 12:47 /Shared Items/Public/
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit


New directories created inside a directory with this ACL will also inherit it. Files created inside the directory inherit a similar ACL: user:_spotlight inherited allow read,execute.  As it turns out, applying the allow list,search,file_inherit,directory_inherit ACL to a regular file results in the file getting the simplified ACL (allow read,execute).  So my solution was to add the inherit ACL recursively to the shared directory:

$ chmod -R +a 'user:_spotlight allow list,search,file_inherit,directory_inherit' /Shared


This solution worked perfectly. Now all my share content shows up in spotlight searches regardless of who owns them.