Radio Split-Screen works with both MkIII and MkIV but the coding is dependent on the radio, not the Nav Computer.
You can code your MkIII (using e.g. NavCoder) to the NG radio (BM54) setting, which will give you radio split-screen mode. Only problem with this is, that the INFO and SELECT buttons won't work any more, provided your radio is a OG model (BM24).
PS NG radios are built in starting from MY2002 (i.e. from 09/2001 onwards).