mincore is a system call and does not take a pid parameter. This means it has to be executed by each process individually, which would require injecting code into each running process and executing it in some way.
Unless there's a tool which makes this extremely simple (maybe Intel's pin?), I believe that writing a kernel module is simpler. The module's init function tallies up the pages and writes out the result into the kernel log. Then the module exits.
You don't need to run mincore() on target pids - you just need to write a tool that opens(O_RDONLY) and mmaps(PROT_READ) each library file, then calls mincore() on each page in the mapping to find out which pages of the library are loaded shared.
The results of mincore() in one process with a shared mapping of a file are enough to tell you how much of that file is loaded shared system-wide.
Unless there's a tool which makes this extremely simple (maybe Intel's pin?), I believe that writing a kernel module is simpler. The module's init function tallies up the pages and writes out the result into the kernel log. Then the module exits.