John, On a bit of a humorous note, if you put a No. 6 brake in the cab of No. 10, you wouldn't have room for one small item...the engineer!
Also, a 6 ET would not be required, as ET stands for Engine and Tender, and our Forneys don't have seperate tenders. A 6 BL might be a better choice. However.... I believe the FRA doesn't require air brakes on equipment that is not used in interchange service. Things like scale test cars have no brakes at all. I don't think they even have a requirement to have brakes at all! However, the Power Boiler Act of 1919 (Federal Law) does state that if a locomotive is equipped with an appliance, it must be in working order and cut in. Just as some railroads choose to not equip their locomotives with a safety control pedal (deadman) the law states that if it is so equipped, it must be in working condition and cut in and working.
Keith