Thats really quite a nasty problem, and its not just reverse engineering ntldr, you would need a heavily customised bootloader to run before ntldr with code to read the mac address out of the network card without an OS available, which isnt easy even if you fix the network card bus / manufacturer. Add to that you need ntlm support to do the meddling, and its not really practical.
You could look at winPE, which is a stripped down copy of windows that is used for microsoft install cds. It can be scripted, has basic network support, and can mount the registry of the host pc. You *might* be able to piece together a script that boots winPE, checks the mac, makes the registry changes, then reboots.
The trick would be having your boot.ini set up to load the PE partition first, do the script, then modify boot.ini back to normal and reboot.
you would then have to have some software running in the host OS to change boot.ini back to point to your PE partition for next time.
Downside is you have an extra soft boot each turn on, benefit is its a lot less work. If its for uni i guess these kinds of things can be explained away as it being a prototype.
Here is a link to a blog post, which links to the download locations for v2, it is free to use, but dont expect miracles by way of documentation
PS: there may be ways of doing this without the reboot, but nothing easy springs to mind
Best of luck,
If you can describe you actual application for this there might be far better ways of approaching the problem.
If you dont need direct hardware access from the XP copy booting, and you can take a small performance hit, you could solve this whole mess in userland code by using a virtualisation technology, such as Vmware or ms virtual pc. You can mount the virtual pcs drive, tamper with the registry, then boot it. That way you dont have to mess with low level hacks, and can spend your time on better things.