The USB drive will be the usb@b/disk@2 device. So it should then be the same as your G4s.
Get the USB device alias with `devalias` e.g. "usb0", then boot with something like;
`boot usb0/disk@2,\\:tbxi`
Most Mac boot discs will load from partition 3, so "usb0/disk@2:3"
Play around with a few variations until you get it. OF feels like trial and error most of the time