... and the iridium uplink, which is presumably expensive, only as a fall-back if normal HF communication is available.
Expensive relative to what? The consumer grade iridium device is only ~$1000.
Per-minute or per-byte charges are also really high. But that's "high" relative to individuals, maybe not compared to operating an airliner.
Something like 3USD/min it looks like. At GSM's 13 kbit/s, that's ~0.4cent/kbit. It doesn't sound like a significant cost to report a position regularly at that rate, not compared with the cost of flying an airliner anyway.
I'm not sure about the details of the plan, but iridium offers unlimited data for something like ~$100/month. I'm sure bandwidth is terrible, but again, these costs are small, even for individuals.