"7z rn archive" (without file names) does the same as "7z a archive"
I cannot help you because I do not have Linux and I do not know how to make a plugin for 7zip.
If you want to make a Windows DLL, you also have to compile and link the source files like CopyCoder.cpp that contain the symbols not found by the linker. I have not done this. Rather, I just added FontHandler.cpp to the makefiles of 7-zip.
Font handler for 7zip