A lye mixture might clean it off, but lye can also etch glass at a certain strength, so hopefully you can get it to a proper dilution to eat away the gunk and then rinse before it has time to etch that carboy.
Does it look hazy, like chemical etching? You said they were old carboys, do you know what was in them before? If they were used in a lab, handling chemicals potent enough to etch the glass, you might not want to use them for brewing.
These were used for winemaking. I tried vinegar and then a strong star san solution and it's barely noticeable now. I am going to try hot water and vinegar soak next.