Sounds like two different approaches being talked about here.
One is having an Emulator, complete and all, installed on a USB stick. This will work, on pretty much any PC.
Having a PC boot from your USB stick, and boot straight into AmigaOS is a different matter. First of all you will need a PC with Boot from USB capability (most modern ones do). Second you need a largish (think >1GB) USB stick and some form of OS.
I am thinking of doing something like this on a cut down version of Windows 98SE. Linux could also do.
But keep in mind a couple of things, this OS is installed with a basic set of drivers, which means it might not work on every machine. Specially networkcard drivers...
Consider that most, of not all USB sticks are marked as "mega floppy drive's", which means an OS will treat it as a removable device, not a fixed disk. To correct this you will need to modify sector 0, try a google search on how to exactly do this.
A simpler option, at least in compatiblity terms would be a bootable CD, much like AROS already is. Works on waaaay more machines. Only drawback would be that your OS is read only....
