Was "乎" the man'yōgana spelling of the accusative/object particle "を"? In the English Wiktionary entry for "" there is a quote or example sentence using the character "" with no explanation seemingly where the particle "" would normally occur. Now I couldn't find anything in either the entry for "" or "" 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 ""?

Yes, it was one form. From here:

> [o] [wo]

**Translation**

In the Nara period, was pronounced as "o" and was pronounced as "wo", and were clearly distinguished. []{}(Manyogana) used , etc. for and for .