Initially, MySQL also supported BerkeleyDB, another KVS, but as for now they dropped the support.
From InnoDB's point of view, MySQL is just a client, as is HandlerSocket the author is using.
PostgreSQL's storage engine is much more closely integrated into the system and has no low-level API easily available from the outside.
How MySQL and InnoDB talk is very much a K<->V / row based relationship.
PostgreSQL supports C-based UDF so it might not be impossible, but I'm not sure whether PostgreSQL has a standard/documented/safe way to access to internal data structures from UDFs.
…and…
Yoshinori-san, very interesting! We use a similar technique for our site together Postgresql. We have our own Software ODBA that writes SQL, marshals the data and puts the marshaled Data into the PGsql-DB. All of our data is always in the memory if the Application is running. ODBA is written in Ruby. You can find it here: http://scm.ywesee.com/?p=odba/.git;a=summary
So maybe?
They said it was actually faster than other KVSes they tried. This was also a while ago.
Here it is: http://thinkvitamin.com/code/steve-huffman-on-lessons-learne...
It's called "Lessons Learned at Reddit", and mostly talks about building and deploying large-scale web applications (the technical, not business side of things.) Well worth watching (there's also a transcript.)