As per the Linux kernel documentation, there are three different system calls to control the memory policy. These calls always affect only the calling task, the calling task's address space, or some shared object mapped into the calling task's address space.
The MPOL_INTERLEAVED mode specifies that page allocations be interleaved, on a page granularity, across the nodes specified in the policy. As mentioned before, those calls always affect only the calling task, so there is no way to invoke them outside of the specific process.
The numactl tool is packaged with the run-time version of the library containing the memory policy system call wrappers. This tool is convenient and flexible in order to run mongod and modify the memory policy for the process.
However, is there any chance for mongod to use the API calls to set the policy without requiring numactl? It will be easier for the users if this can be done in a transparent way.