You could have an initial code segment which generates the dynamic mappings and then performs a system call. The system call would map in a new non-shared page

You could have an initial code segment which generates the dynamic mappings and then performs a system call. The system call would map in a new non-shared page which the kernel has dynamically generated from the supplied mappings. The user-land never sees the page as writable, only as executable, the kernel generates only jump instructions, and limits this behaviour to specific pages determined when the executable is loaded.