The FDSN code for rotational data is unofficially "J", likely to be formalized in January, 2009.
The code "J" has tentative approval by the FDSN WGII. It will be officially approved at the next FDSN meeting in January 2009.
It would be safe to use "J" for the instrument code, but documentation won't be updated until Jan 2009.
See http://merkur.geophysik.uni-muenchen.de/pipermail/rotation/2008-September/000040.html for more information.