Comment by PhilipRoman

Maybe that wasn't the best example since you cannot even cast to a function pointer within standard C (but you can in POSIX). If you consider POSIX as portable then writing your own dlopen is pretty simple and you don't even need to be aware of calling conventions, though there are some architecture specific things in the ELF format. But either way it will not integrate well with mechanisms that people expect to work like LD_* environment variables, /etc/ld.so.conf and so on. So I still think it matches the "possible but will not integrate with rest of the system"