Yes -it's possible, although I think that the Bag Drop desks aren't supposed to do it. Only the Check-In Assistance desks. I've had it done for me at Customer Service at various outstations, and the Flight Connections desks. It is also possible for them to do it at the gate, but again I think it is policy for them not to do so to speed up the process.

Also, the telephone agents can cancel your checkin somehow. I don't know if this then allows you back into OLCI or if you would then get the message telling you to proceed to the airport, but I've had my checkin cancelled a few times when making changes to a flex ticket after I'd checked in and printed BP. It does take a bit of time for them to manually process IME.