My feeling is that if you genuinely find an onomatopoeia that works better than existing ones, you can certainly use it on the fly. This is really hard though, as a) there are already so many of them, and b) there are unwritten rules that restrict possible onomatopoeia that will communicate successfully.
Children often do create new onomatopoeia, but if ...
Sorry I can't answer your question directly.
However, there is a great deal of sound symbolism in Japanese. Some authors coin new onomatopoeia for translating foreign concepts, or utilize onomatopoeic affixes just like Kango affixes (zazamushi was coined with zaza symbolizing the sound of waterflow, and mushi meaning "insect") .
This ひら doesn't mean 平(flat, plain) but ひらり.
Dictionaries say ひらり means
ひらっと is almost the same as ひらりと, which means "lightly", "nimbly".
「ひらっと」 is not a very common onomatopoeic adverb to use in the context of passing by someone. It would, however, mean "casually", "quickly", etc.
More commonly, you would encounter 「さっと」、「さーっと」、「すっと」、「すーっと」、「ふらっと」, etc.
Not sure what dictionary you use, but 「ひらっと」 just could not mean "flat or "plain" in the context. You would not pass by someone "flat(...
I think the most common (and versatile as well) one for us native speakers would be:
「パッと」 is not a bad choice, either. It just sounds lighter and more conversational than 「ふと」.
EDIT: 「はっと」 is also a common one.