I'm sorry if I fail to understand this... When you say, "it doesn't have to be your real domain" do you mean edit the template so we display one domain and then set the variable to the real domain in the template code?
You can set:
$wgLDAPDomainNames = array( "mydomain" );
To any thing you want. It is shown to the user (or not shown in your case), and used for the options that proceed it, but it doesn't matter how you set it. It isn't used for authentication to the domain, and doesn't have to match your real domain name.
Thanks for your LDAP integration work by the way.
No problem. I'm glad you find it useful.
V/r,
Ryan Lane