POSIX specifies that setting SA_NOCLDWAIT in sigaction.sa_flags for SIGCHLD has the same effect on wait* behavior as using SIG_IGN for sigaction.sa_handler for SIGCHLD. So, by POSIX rules you can just as well use SIG_IGN if you do not care to actually handle the signal.
However, the Hurd does not support those semantics at all. So, if valgrind really depends on that behavior, it needs to be implemented first. That requires support in the proc server, including a new call in the proc RPC protocol, as well as support in libc.