NFS and UFS Mounting on a non-empty directory

[Gregg Kishida]

I am currenly managing a variety of systems, all of which seem to be
in one way or another incompatable with each others implementation of
yellow pages (YP).  Due to disk space limitations, they need to serve 
from eachother via NFS.  At any rate, it would be nice to mount a password
file remotely via NFS instead of using YP.

However, it seems to me that almost certainly the passwd is necessary to
have around at boot time (i.e. I need to log in as root if the server dies).
The big question is... is the concept of NFS mounting on a non-empty directory
well defined???  Could I place a default passwd file in a directory, and
later mount the NFS one on top of it safely.. (would the results be the same
between various NFS implementations.... ie.. SUN 4.0, DEC Ultrix, HP UX, Xenix,
IBM A/IX)  (yes, we have at least one of each).

Thanks in advance.
Guy