A more feasible solution might be to slightly alter the somewhat clumsy tone marks used in Cantonese For Everyone:
1: āa / ā / ē / ī / ō / ū / ōe / ēo / yū
2: áa / á / é / í / ó / ú / óe / éo / yú
3: aa / a / e / i / o / u / oe / eo / yu (discards the unintuitive and difficult-to-type circle diacritic)
4: àa / à / è / ì / ò / ù / òe / èo / yù
5: ǎa / ǎ / ě / ǐ / ǒ / ǔ / ǒe / ěo / yǔ
6: âa / â / ê / î / ô / û / ôe / êo / yû (replaces the impossible-to-type non-Unicode-compliant elbow diacritic)
This system is rather similar to Hanyu Pinyin, which could be a benefit for many students. Either way, reasonable tone marks are often more intuitive than superscript numbers.
1: āa / ā / ē / ī / ō / ū / ōe / ēo / yū
2: áa / á / é / í / ó / ú / óe / éo / yú
3: aa / a / e / i / o / u / oe / eo / yu (discards the unintuitive and difficult-to-type circle diacritic)
4: àa / à / è / ì / ò / ù / òe / èo / yù
5: ǎa / ǎ / ě / ǐ / ǒ / ǔ / ǒe / ěo / yǔ
6: âa / â / ê / î / ô / û / ôe / êo / yû (replaces the impossible-to-type non-Unicode-compliant elbow diacritic)
This system is rather similar to Hanyu Pinyin, which could be a benefit for many students. Either way, reasonable tone marks are often more intuitive than superscript numbers.