No problem; because all of the machines are in your domain you can put together a VBScript that will get the MACAddress(es) from the local machine and store it as an attribute of the computer object in Active Directory.
Here's a quick hack on how to do that (save this as a .vbs-file):
Const ADS_PROPERTY_UPDATE = 2
Const COMPUTERLOCATION = "ou=Member Servers,dc=yourdomain,dc=com"
Const ATTRIBUTETOUSE = "otherTelephone"
Dim wshNetwork, strComputerName
Set wshNetwork = WScript.CreateObject("WScript.Network")
strComputerName = wshNetwork.ComputerName
Dim objWMIService, colNetCards, objComputer, objNetCard
Set objWMIService = GetObject("winmgmts:\\" & strComputerName & "\root\cimv2")
Set colNetCards = objWMIService.ExecQuery("Select * From Win32_NetworkAdapterConfiguration Where IPEnabled = True")
Set objComputer = GetObject("LDAP://cn=" & strComputerName & "," & COMPUTERLOCATION)
For Each objNetCard in colNetCards
objComputer.PutEx ADS_PROPERTY_APPEND, ATTRIBUTETOUSE, Array(objNetCard.MACAddress)
Because your clients aren't all in the "Member Servers" OU above you'll need to modify the above script to include a directory search for the strComputerName do get the COMPUTERLOCATION.
When you have a working script, ask your domain administrator to put the script as a start-up script targetting the computers you need to monitor; that way it'll execute whenever a computer boots up. You can also run the script as a scheduled task to get your data from any clients that haven't rebooted or use psexec or some other way you can think of to get the data immediately. Or you can rewrite the script entirely to remote connect to all of your machines and get the data that way (which might not be possible due to local firewalls). Or you could write a small .NET console application which does the same thing, it's up to you...
Also, although there is a networkAddress-attribute defined for computer objects; by default the computer object itself does not have access to write to this property. Because start up-scripts run in the context of the SYSTEM account on the particular machine the easiest thing is to use an attribute that the computer object (SELF) has write access to. The otherTelephone-attribute is multivalued and part of the Personal-Information Property Set which all computer objects has write access to by default. If you want to use the networkAddress-attribute you need to set explicit write access to that attribute for all of your computers.
Also you need to bear in mind that storing the the MAC address in Active Directory means that all of the users in your domain will have read access to it which in turn might possibly (depending on your environment) pose a small security risk.