In the English Wiktionary entry for "を" there is a quote or example sentence using the character "乎" with no explanation seemingly where the particle "を" would normally occur.
Now I couldn't find anything in either the entry for "乎" or "を" about them being historically connected via man'yōgana, and Google searches failed to turn up anything conclusive either.
In fact I don't know much about man'yōgana at all so could it be that any character with the right reading could be used for "を"?