Apparently, this is now an Android SELinux security feature -- mounts initiated from most processes are not visible to other processes. See [this answer](http://stackoverflow.com/a/29727734/2975337).

A related [Android kernel development thread](https://groups.google.com/forum/#!topic/android-kernel/JeY_u-pHcxQ) suggests replacing an uneeded system service (e.g. `/system/bin/debuggerd`) with a shell script that runs the desired mount command. This you'd launch with `start debuggerd`.

This did not work for me. However, I simply added my mount command to the end of `/init.rc`, and the mount was visible system-wide on bootup.

If you have `init.rc` and want more flexibility, you *might* be able to launch a daemon from it to execute mount commands received via IPC. But I'm not familiar with SELinux's house rules.