Closed Cell pad options

Canada's equivalent of REI is MEC (I think that's Mountain Equip. Coop). If there's one close to your destination, a blue foam pad isn't too expensive to buy there, and then just donate to someone at the trailhead if possible when you finish.

You could also mail one to yourself.

Paying for baggage is an abomination…

Gossamer Gear has the Thinlighs, pretty warm and tiny. No padding, but pretty warm and tiny.

Check the regulations. Could you carry a stuff sack, same as a woman's purse, with the pack as your carry on?

Would that free up enough space for the pad to go in the pack? Sea to Summit made a silnylon, lengthwise zippered bag/stuff sack with sewn loops on each end. With micro 'biners in each loop, and a strap through the 'biners, it carried like a woman's purse.

I often carry a large shopping bag as my carry-on that could easily hold a sleeping bag plus a few other things. Then as soon as you get off the plane, strap your pad to the outside of your pack.

Zack,
Yes! That is exactly what I ended up doing and it worked perfectly. I easily carried a pad along with a crazy creek chair in the bag.