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