Actually, that is incorrect - you can do it at OLCI time and request it in the AC OLCI interface.

Even if you are flying from somewhere that AC does not operate e.g. XXX - FRA - YOW you can start the check-in process using FRA - YOW and request the u/g and then bail on the check in and the u/g will still be in place and then go OLCI with the carrier that is starting you off in XXX.