Bypassing SQL lexer and parser

Hello!

What is the best starting point to PostgreSQL internal APIs for operating directly with the storage (performing basic INSERTs, UPDATEs, SELECTs and simple JOINs by hand)? I'm looking for something similar to MySQL Cluster NDB API or InnoDB internal API (the late HailDB and Embedded InnoDB).

Does the PostgreSQL support any other type of plugins/extensions other than FDW and custom data types? I mean, is it possible to start another daemon within Postgres without slightly modifying the main codebase?

In case you are wondering why could anyone need something like that: I'm looking for a way to implement a small subset of HTSQL (see [http://htsql.org/], of course not with the whole HTTP protocol) natively in one of popular RDBMS without extra URL-to-SQL conversion. Yes, I know that a lot of hard work was done for making query plans, optimizing etc. but I'm still really wish to do this for some very specific needs. I'm not going to completely replace the SQL and thus will be happy to do those manipulations on a SQL 2008-compliant DBMS.

Thanks in advance,
dannote