Re: RID master
DO you then cluster the RID master over geographic locations to ensure redundancy? Do you then need a RID master master to oversee your RID cluster?
How will the ID blocks work when they are re-merged, the transactions will be all over the place? If you don't need the ID for anything useful outside of a unique index then you could just do a compound index with the server name or start your index block at different starting points that will never overlap.
In reality it isn't so much about inserting data once or reading data, it is about changing data or a set of transnational commands that needs to be done in a set sequence where some of that sequence may exist on one server and some on another and where the times could be ms out. Or where some data is changed that only exists on one system, or has only made it into one index.