If I _were_ to modify /sbin/mount and umount to use an mtab file for
   recording mount options and fstype thus making umount_* possible.

That would be undesirable.  As you pointed out yourself, there's
another solution to this.  The reason for keeping this info in the kernel
is so that it's always correct, even when root in mounted read-only.

BTW, I'm a little confused.  Are you having inetd fork a separate
process for every mount, and creating server processes when a mount is
requested?  If that's the case, you could instead have a master daemon
that accepts both mount and unmount requests, forks the servers, and
remembers which processes are serving which mount requests, so they can
be shut down later, using a signal or whatever.