"you would rarely encounter actual written Hiragana" is outright false. It is nearly impossible to write a full grammatically correct sentence in modern Japanese without the use of Hiragana, as Hiragana are used for subject and object markers, conjugation of verbs, question and assertion markers, possessives, adjectives, negation, and many many more grammatical constructs.
Source: read literally anything in Japanese, like an article from today's news https://news.web.nhk/newsweb/na/na-k10014946221000
You could be describing a rotoscope or a modified version of one https://en.wikipedia.org/wiki/Rotoscoping