Here is my SDCC library for accessing plus hardware.
I used it in the cpcwiki rom that I made.

pluslib.h is the header. you can build the library from linux or from windows and use the pluslib.lib.

Included are functions for setting colours and setting up sprites.

For other functions (setting mode, reading keyboard, then you will need another library).

To use, you will need to unlock the plus hardware, and call these functions. Please ensure these library functions and/or your code are not in the area &4000-&7ffff, this is where the plus registers are. If your code exists in the same place, you will see problems.

I will add more functions when I have time.