Can I use SQL Relay to make my existing MySQL app send all insert/update/deletes to a master database and distribute selects over a set of slaves?
Probably. As of version 0.38. You’ll need to set up SQL Relay to route queries and use the Drop-in Replacement Library for MySQL to redirect your app to use SQL Relay. See Routing and Filtering Queries with SQL Relay for details on how to set up SQL Relay as a query router. See Using the SQL Relay drop-in replacement library for MySQL for details about how to use the drop-in replacement library.