Perhaps the real solution here is to let non-root use seccomp-bpf.
Non-root can use seccomp-bpf. Chrome already does.
So theres nothing preventing OP implementing what they want entirely in userspace with no kernel changes already?
malloc can use mmap (aside from s/brk) - it's impossible these days to sandbox any more complex program and not allow mmap (arguments can be limited though to only a subset of flags).
But that would be the same for both this proposed mseal() and my proposed seccomp solution.