Apparently, this is now an Android SELinux security feature -- mounts initiated from most processes are not visible to other processes. See [this answer](https://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`.

**Update:** This did work for me, after a reboot.