Stumped. How to set colspec/@colwidth units?

In a CALS table (in DITA), the colspec/@colwidth value is written with an "in" suffix. I'd like to use other units (like "mm"). For a frame, equation or facet, you can use the r/w rule of ..

     specify size in mm;

.. to set the units. But this doesn't work for tables (or the colspec element). I've tried setting the global units in the document properties. It shows as "mm" in the UI for setting a column width, but in the XML it's always written out as "in". FM will properly read colwidth values other than "in", but on save they always go back to "in".

Have I completely broken something that should work, or is it not possible to set units other than "in"?

Thanks!

…scott