iMessages aren't converted to SMS on Apple's servers side. It's a fallback on the phone's side. When you're on a WiFi but with no cell coverage, you can send an iMessage but can't send an SMS.

But, for such a satellite phone service, Apple totally could make a SMS to iMessage gateway, or even just a web interface where non-apple users could log in and see that someone had sent them an iMessage.

Obviously Apple will make the process as painful as possible to encourage buying iPhone.

This isn’t entirely true. If you have WiFi calling enabled, you can also send SMS over WiFi if the carrier supports it. I use this extensively when traveling internationally.