I need to set the address where the library I've included will reside in the code memory
I'm concerned to prevent the user mode from accessing the other regions. Then I suggest you put those other parts of code into a decidated region, which you can configure as system-mode only.
But before you make any further step, you really need to sit down and learn a lot about how the '51 tools work. This is a highly unusual C toolchain, for which a lot of things that would apply cleanly on almost every other system, just don't.