That expiring triggers could be somehow useful.
But in most cases it's not worth to query memcached from database (selects). The reason is it's much faster to query memcached directly without even opening database connection.
All these caching things make applications complex, because cache invalidation it's a complex task.
MySQL is doing right steps - it's doing caching itself via InnoDB buffer pool. And makes the interface faster by replacing SQL queries with fast memcached API.
"The Memcache Python API
High performance scalable web applications often use a distributed in-memory data cache in front of or in place of robust persistent storage for some tasks"
MySQL memcached is not a cache, it's a persistent storage with memcached API.
And it's fast.
All these caching things make applications complex, because cache invalidation it's a complex task.
MySQL is doing right steps - it's doing caching itself via InnoDB buffer pool. And makes the interface faster by replacing SQL queries with fast memcached API.