It's easiest if you can use that one letter, possibly with a modifier key such as shift or caps lock. (It depends on your scanner as to whether including modifier keys is possible.)
I've set up one scanner to be triggered with shift-caps lock ~. That's one character with two modifier keys.
The script doesn't have to interpret any words, though, it just puts the cursor in the correct text field to receive the rest of the scanned data, then a second trigger is tripped by text at the end of the scan (I prefer tab, enter or return to trigger onObjectExit) to process the data that no sits in a global text field--either looking up data or creating a new record.
You could use a onLayoutKeystroke to detect a word by appending each triggering keystroke into a global field or variable, but why would you want to do this?