Hi @serviceberry
From a HW viewpoint (I am HW team) that sounds reasonable to use the hub the way you suggest. But I know for a fact from personal experience the Boson's are very dependent on VBUS rise times, and a very fast rising 5V supply may not allow it to enumerate properly. We have had an instance in the past where one particular VBUS port rose really fast, and the Boson did not enumerate until we slowed it down. It's one of those situations that may require testing to check. Our J3 10-pin host port has a nice moderate rise time that allows Boson to enumerate. So, when using a Hub, check the VBUS rise times compared to J3.
I do NOT expect J9 USB-C OTG port to work with Boson.
The Snapdragon SOC is very smart, and when in OTG mode, it will not provide more than 500mA to any device that enumerates as USB2 which I am lead to believe how the Boson enumerates. if the Boson could enumerate as a USB3 device and the drivers were all present in the Kernel for this, the Snapdragon will give it 900mA over VBUS letting it boot but I think this is a non-starter without the Boson being able to do that.
Our Boson kits use USB2 only and we provide 1A support to enable them to work without requiring enumeration to negotiate more current, hence why they work on the J3 ports and other expansion board ports that have more than 500mA: https://docs.modalai.com/expansion-design-guide/#usb-expansion-over-j3--j5