At Arck one of our Elgg clusters is configured in a similar way accept that we use two load HA proxy balancers configured to failover using keepalived. The load balancers also handle all of the TLS negotiation. The database servers are configured active/passive and use pacemaker to manage the failover of the databases. This setup has served us very well for quite a while however I find pacemaker a little overly complicated and so we are testing running on a 3 node Percona XtraDB Cluster (multi master). This requires that all the tables be InnoDB which is not a problem for Elgg. We generally offload all of our search to a SolrCloud cluster however MySQL 5.6 full text searching is available in InnoDB so this setup should work well also. 

@Billy, thanks for your feedback ! I was planning to have the application servers do the TLS handling, but that should not be any issue.

I guess you want Percona for the multi master option, for now I was just planning to use MySql cluster software from Oracle itself.

Eventually if this site really is going to be a huge hit, I want to move to Amazon and use their dynamic scale out functionality.