If you only want the username and not the domain why not validate as early as possible, e.g. don't all \ in username. As always have server validation to back this up in Login action like you suggest, e.g.
if(model.UserName.Contains('\')
model.UserName = model.Username.Name.Split('\')[1];
If it is always the same domain, put the domain name beside the text box to make it clear it is not being asked for.