You can't determine from metrical feet if the 'i' in 'cuius' was pronounced as 'j' or if it formed a diphthong with 'u'. Diphthongs always form long syllables so it could be the 'u' was long and the 'i' was pronounced as 'j' or it could be that 'ui' formed a diphthong. Do we have any other way of knowing this information? Further, most modern dictionaries assert that it should be pronounced as 'cujus', well, why do they believe this?
Also, there are the few times when Latin got transliterated into Greek. Does anyone know if 'cuius' or other such words were ever inscribed in stone in Greek?