This is a really interesting discussion, since the model we are describing resembles closely to HandlerSocket Plugin (since it is build upon Handler API instead of Storage Engine) it led me to research some more about Benchmarking test involving Memcached + InnoDB vs HandlerSocket.
With MySql version 5.6 Benchmarking test had given to significant advantage to HandlerSocket (processing 750,000 Qps) over Memcached InnoDB plugin (420,000 QPS). here:
But, with in the recent test made my MySql team on MySql version 5.7, there was significant improvement with Memcached - InnoDB plugin now able to process (1,000,000 Query/sec (QPS)) , so in my view point it will be better to work with direct Storage Engine access instead of building it on top of Handler API.
Also as an excellent point raised by Piotr making Memcache Plugin will duplicate of HandlerSocket plugin, but speaking different wire protocol , which definitely won't give us much advantage .
Will be happy to hear any counter questions/points about it.
--Aalekh