The Zhuang people of southern China have been using an augmented Chinese script for over 1300 years called Sawndip. This writing system was used extensively in popular culture (songs, poems, ceremonies, and some literature) and religion, but not governmental documents.
As with Chu nom, some of the Chinese characters are used “as-is” (about 80-90% for Sawndip), but the Zhuang also developed characters of their own using the same mechanisms for composing characters as Chinese script uses. Most often, they combined one character for the meaning and one character to give the pronunciation. This is tricky, however, since Zhuang has sounds that do not occur in Chinese. Frequently, the phonetic radical’s Chinese sound is only a very rough approximation of the sound in Zhuang.
This, coupled with a lack of government-imposed standardization, meant that there was a huge variation in the characters. Indeed, it is difficult to read Sawndip manuscripts, and it appears that they served more as a mnemonic device for the manuscript’s owner than as something just anybody could pick up and read. (I found one source that indicated that priests didn’t always understand what they recited, just how to recite it.)
In the late 1950s, the government of the People’s Republic of China decided to standardize the Zhuang writing system with a phonetic alphabet. (Why they decided that a minority language should be phonetic, while it was clearly just fine for Chinese to use logograms, was not explained in any of the supporting material that I saw.) At first, they used this strange bastard love-child of Cyrillic and Latin script, but in 1986, they stripped out the non-Latin characters.