Based on my using Google Messages many times on AC flights, this is exactly how it works. For the less tech-y, with other Android/Messages users, my sent messages are a dark teal colour and I can continue to send and receive them anytime I have Internet access, including on AC WiFi. For those using iPhones or whatever, my sent messages show up as light grey, and if you tap on them will say "SMS" in the bottom right below the message. Not using dark mode, I think the non-SMS messages should still be dark teal, while the SMS would be a light blue instead of light grey.

Since AC has said Google Messages will work with the free messaging plan, I assume my above experience - using paid WiFi - will also be applicable to the free messaging plan.

I don't know what to make of the cow's comments above about Google Fi. I'm just using a regular boring data plan from one of the Canadian telcos. I'm on a Samsung and have Messages set as my default SMS app; I don't know whether it's any different if you use some other app as your SMS app.

What I'd like to know is whether Signal works. I've used it on multiple airlines with free messaging plans where it wasn't one of the apps listed as being available, but not sure whether to read anything in to its absence from AC's list.