I have been travelling frequently to/from Canada, and I never ever could change the frequent flyer number on a partner issued award ticket on AC metal, and know at least 5 other people who tried and failed. It is not possible to do it online, and over the phone or in the airport agents cannot (or maybe do not want to) do it.

Can you please point me to how you did it online?
On AC website after retrieving the booking, all options to change/add info are greyed out, and it says on the very top "This booking appears to have been completed elsewhere than on aircanada.com (with either a travel agent or on another website). To view your complete booking, or to make changes to it, please contact your travel agent or return to the website on which you made your booking." This has always been the case with partner issued award tickets on AC metal.