Solved one of my issues, I was attempting to include chip.h in the lpc_core directory in order to get access to the LPCOpen API, but this would generate a lot of redefinition errors. Instead I can do the following:
#include <lpc_types.h>
#include <gpio_112x.h>
this gives access to the gpio API (the types is necessary, otherwise keywords like STATIC are unknown).
I am still curious if users in general use the LPCOpen library or not.