Accessing a custom FileSystem (as in Mysql "Custom Engine")

Hi,

I have a custom "database" (a very fast select/slow insert db) written
in c/c++.
I can access it with mysql writing a "Custom Engine". That is pretty
cool because now all my "custom db" tables can be joined with tables in
mysql's format.
I only need read access to my custom table format db. Would read access
be possible with the "C-Language Functions - Returning Sets" API? I
would like to read my custom tables and join them with postgres tables...