This is still a problem with the current standalone.

    \definelabel[X][text=]
    \startTEXpage[offset=1cm]
       \X\X[a]\X[b]\X\par
       \convertnumber{R}{\ref[number][a]}
       \convertnumber{G}{\ref[number][b]}
    \stopTEXpage

    %% The above example fails to compile when there is no tuc file,
    %%    complaining of a missing number in line 4. When lines 4
    %%    and 5 are removed and the file is compiled, producing
    %%    a .tuc file, and then they are restored, the example
    %%    compiles without error. The effect of having or not having
    %%    an associated .tuc file does seem a bit odd.

Surely there is a way to get the number from a label reference that can 
then be converted to a numeral of choice.

-- 
Rik