I just found it. It was more difficult than it ought to be. There is no orchid entry in any of the Paiwan dictionaries I can find online. Most of the entries for flower lists the Japanese loanword hana. I even saw an entry for orchid in a speech of a student which seems to list it as maretimaljimalji.
Finally, in a Tainan city Paiwan textbook, it lists 3 words for flower, hana, vuciar, and Veljangaljangaw. That ljangaw part must had been the etymology for Long-kiau. If that’s the case, then what Le Gendre wrote down on his maps, Liang-kiau is a much closer phonetic transliteration.