Iridium would work fine for this if you limit it to iMessage/SMS and anything else low-bandwidth (I would imagine they add 911/SOS support too).
I have a Garmin InReach which operates on Iridium, it's a 5+ year-old-device which works fine with Iridium in very challenging conditions and isn't very much larger than an iPhone.
Considering (some of) the commercial side of this, I strongly suspect the Iridium network has more than enough capacity for such a plan and Iridium could easily support it technically and they could work out some reasonable commercial terms with Apple for this. If Iridium doesn't want to work with Apple the company could surely be acquired for effectively pocket change by Apple and if they did release such hardware and charge a monthly subscription for "100% global coverage" it could be pretty quickly profitable.
But, with all of that said, I'm really not sure why Apple would want to do this. Like I said, I have an InReach, I'm commonly in areas with no cell service and so for me this would be a clear win and I would love it, but, I suspect I'm squarely in the minority.