Problem solved!

Bennugd.elf is hardwired to look for a file called "mass:/main.dcb", but this can be changed with a Hex editor!
1) Open up bennugd.elf in a Hex editor
2) Search for the string "main.dcb"
3) You should see a piece of text that says "mass:/main.dcb"
4)
Make sure your hex editor is in overwrite mode. It is critical that the new file name you put in is
exactly the same number of characters as the old file name. "mass:/main" = 10 characters. Don't add any extra characters or take any away. Keep it precisely 10 characters.
5) Change "mass:/main.dcb" to "0123456789.dcb". (you can choose any new filename you want as long as it is 10 characters long)
6) The old "absolute" file location has now been changed to a relative location!
7) Save the changes in the HEX editor. ie File > Save As. Call the new file "bennugdFixed.elf"
8 ) You can now put as many different Bennu games on a USB stick as you want, just as long as each one is in its own folder.
9) Rename your compiled Bennu script to "0123456789.dcb" and put it in the same folder on the USB flash drive as "bennugdFixed.elf"
10) Run bennugdFixed.elf. It will look for a file called "0123456789.dcb" in the same folder as itself! This means you can have as many different Bennu games on a USB stick as you want now, as long as you put them in their own seperate folders.
bennugdFixed.elf and 0123456789.dcb must be in the same folder as each other.