Well, the way /usr/bin/units solves it is to allow "floz", "usfloz", "fluidounce", or "usfluidounce".

Having to learn that you can type "floz" is better than not being able to do it at all.

(You could also allow "fl oz", "fluid ounce", "fl ounce", etc. if you tweak your parser a little bit, but that's the deluxe version, and any functionality at all would be a big improvement.)