User tag design for read-only index

[Ryan McKinley]

I am build a system that indexes a bunch of data and then will let  
users manually put the data in lists.  I have seen http://wiki.apache.org/solr/UserTagDesign

The behavior I would like is identical to 'tagging' each document with  
the list-id/user/order and then using standard faceting to show what  
lists documents are in and what users have put the docs into a list.

But - I would like the main index to be read only.  The index needs to  
be shared across many installations that should not have access to  
other users data.

Any thoughts on how this might be possible?  Off hand, it seems like  
manually filling an un-inverted field cache might be a place to start  
looking.  Perhaps using a multi-searcher and keeping two indexes --  
that seems like a lot of work.

thanks
ryan