Not a trick question, I wasn't quite getting what you wanted to do.
Ok, so you could have the defop repl store the current user in a global variable which you could then access from the prompt. Which would work fine as long as you only had one person logging in as an admin at a time.
If you have more than one person logging in as an admin at a time and you want to know who you are from the prompt, it sounds like the challenge is to get the dynamic value of the user name into the scope of the eval'ed expression?
Right, well, notice that the repl-history*, that, and thatexpr are also global variables, and so future proofing it for multiple admins will take more work than just the eval issue.
(For getting values into the eval, one way to do it is with an MzScheme feature called parameters; I have a library to let you use those from Arc which I haven't quite finished yet).
