Hi Ian!
Find my replies embedded below:

I am beta testing ADSM V3 for NT.
Is there anyway to force client PC's to use a DSM.OPT file that is
located on a server? rather than having separate DSM.OPT files sitting
on each client PC,
        I don't think \\servername\share\dsm.opt is supported, but you can make 
a
network mapping (let's say drive X:) and use the -optfile=x:\dsm.opt
parameter. However, I wouldn't do this. You create a single point of failure
here. If the server, which is mapped to, is unavailable all your backups
will fail!
If you have got a version 3 server you can better make use of the 'client
options' feature. This allows you to force most client option parameters
from the server on to the client, which allows you to make your local
dsm.opt as simple as possible, maybe just containing the server address.

However it seems that if I want to backup my registry I need to leave
the NT workstations at the
'logon' prompt. (using DSMSCHEDULE, and running as a service on NT), and
thus would not even have access to a server copy.
        Correct. If you are logged on to the machine the registry files are 
locked
and thus unavailable to ADSM. Just a Windows NT design. You can however make
a online backup by using the Backup Registry feature through both the GUI
and the commandline interface. This copies the registry parts to a local
directory called c:\adsm.sys and then backs them up. Check the Using the
ADSM for Microsoft Windows Backup/Archive Client manual for details how to
backup and restore the registry.
Kindest regards,
Eric van Loon