M17 is possibly what you're looking for? Not specifically mesh but could be used similarly or extended. It's a a protocol that provides data and digital voice transmissions and works (with some modification) on a few existing commercial grade handheld radios running OpenRTX open source firmware. The only thing we need is a radio that has a Bluetooth module.

I sometimes think about buying one of those LoRa arduino shields for some experiments. What exactly is proprietary in these?

The chipset, the modulation, the protocol itself, it's all proprietary and patented.

Range is lower for those though I thought.

Maybe HaLow is more open.