In Fraser, Geen and Scott's The GWR in the 1930s, vol 1 there is a picture (18) which includes a C3 and in vol 2 there is a picture (214) which also includes a C3. Both pictures apparently post date 1933. According to Harris, C3's were built 1891/2 and some were built as BG/NG convertibles, so must have been over 40 years old when photographed. Whether the particular coaches photographed were converted from BG or were NG from the start, I don't know, but to go back to the question posed by BorderCollie at the start, regarding whether GWR broad gauge convertible carriages could have been seen in 1929, the answer is coaches of the same type certainly could, but not on crack services.
PS Identification of the coaches is what is stated in the books, it is well beyond my expertise.