Jacek Piszczek has written a special guide containing instructions on how to boot, install and update MorphOS from USB media. All interested users will find it in the MorphOS website's guide section.
Nice guide. It's been so long since I've fired up one of my MorphOS systems but I'll be trying this out just for the heck of it when I do. Using this technique to prepare the flash drive and expose the correct Openfirmware device is something I doubt I'd have found by myself.
This is a very good reference. Much better than trying to remember exactly how I got it installed via usb the last time around, every time a new iso comes out. Thanks Jaceck.