You've probably also not found a lot of info or help on the web for setting this up.

Hopefully this site will help you with this.

For now you can start where I did, an excellent article on (Tom's Hardware Guide | Windows In Your Pocket), pay close attention to the section "RAM Disk For Boot Problems".
You will need to configure your USB Boot device using this method if you want to ensure it will boot on as many systems as possible.
When you get to the section on formatting your usb device you may want to look at my "Alternate Instructions" page.

The problems with this configuration is that is loads the entire cd image into memory, requiring more and more memory for each utility you add to your configuration.
So I figured well why add the utilities to the image at all? I can boot from the image but store the utilities on the same usb device I am booting from.
Well this requires changing shortcut links and directory locations etc.
This creates a challenge as the drive letter of your usb drive will change with each system you plug it into depending on how many drives/partitions are configured on that system.
I have been able to configure Sherpya's XPE with shortcuts that point where they need to and it seems to work well.
The GeoShell project is still in the works, (haven't been able to get the shortcuts to update)

The Sherpya configuration so far works best but needs the most memory (about 264MB) vs. (about 224MB) for the GeoShell version.
Most newer system have at least 512MB so either version should work. However if you think you will be using it on systems with 256 then you might want to go with the GeoShell version.
You can also burn the BartPE ISO to a cdrom and boot from it and use the usb drive just for programs.

I will be posting more detailed instructions and plugins as I have time.