I would suggest saving the username in a cookie.
in reply to NTLM DOMAIN\USERNAME
If the cookie does not exist, pop-up a form, asking for the user name (One time only), then save it in a cookie, hopefully with a fairly long expiration time.
You should also provide a "logout" option, to allow the user to delete the cookie, so you can do tests and impersonation.
Since you are specifically not asking for authentication, this method is specifically NOT secure, but hopefully meets your requirements.
"As you get older three things happen. The first is your memory goes, and I can't remember the other two... "
- Sir Norman Wisdom