In many place and people names, there is an "invisible の", e.g., 三宮 is read さんのみや and 中大兄皇子 is なかのおおえのおうじ. This can also happen in words: 班田収授法 is read はんでんしゅうじゅのほう, which I am less comfortable with because I suppose if it were modern Japanese it would be read without the の.
As I recall, this is because の was originally not written in 漢字 in olden days, so not all the phonetics were captured in writing (to the extent that phonetics were captured in 漢字). My question is: could someone in olden days know or reasonably guess such readings without having specifically learned these names/words? If so, can anyone explain what the basic rules/guidelines are? (You can restrict this question to being about insertion of の's, but I would be happy to hear about other aspects as well.)