When reading cross-domain, even though the account is an admin you will come up with issues reading the event log. The way I see it you have 2 choices:
1. Impersonation: You can write code that does impersonation. I did this in C# and then had my app impersonate an account on the other domain before making the logparser call. However, since you look like you are calling this from sql server it will be difficult to do that unless you are on sql 2008.
2. An option I did not do, but can work in your case is to use pass-through authentication. Basically, create a local account that has access to read the log on that local box. Then, create the same account on your current domain (the one you are running the command from). When you run the xp command, make sure you use that account as the context...it should now work. You can read up more on pass-through...just google it.