U+0385 (GREEK DIALYTIKA TONOS) is currently mapped to A5 EC (maps to Unicode YEN SIGN and COMBINING ACUTE ACCENT) in CP1258. However, based on the cannonical decomposition specified in unicode, the conversion should most likely be A8 EC (DIAERESIS and COMBINING ACUTE ACCENT). Note that this mapping is already used for U+1FEE (GREEK DIALYTIKA AND OXIA) which is specified to be equivalent to U+0385, confirming the mapping to A8 EC.
I checked in a patch.