RIS installs and environmental variable

I have a W2K RIS server setup. I deploy Windows XP with it. All is well.
When a computer needs to be formated and reinstalled I can boot of the NIC
and this time I can choose Automatic setup as the type of setup and it will
grab from Active Directory the right computer name that it should be. This
is becasue it looks like in the "rinsndrd.sif" file there are a lot of items
with environmental variables that are getting pulled from the server. What
I see so far is:

%ServerName%
%MachineType%
%InstallPath%
%OrgName%
%MachineDomain%
%TimeZone%

What I am wondering is, are there other variables that I can create or
manipulate on the RIS server? For instance, there are unused attributes in
the AD Schema. Could I use one of these unused attributes to store the XP
Keycode of each computers so I could call an environmental variable for the
keycode rather than have to type it in?