He has used asses before. And let's not forget unwitting prophets such as Caiaphas
John 18:14
Now Caiaphas was he, which gave counsel to the Jews, that it was expedient that one man should die for the people.
First of all, I didn’t put any limits on the Lord, I simply asked a man who is claiming to be a prophet a question.
Now, as for your examples, being foolish is not a sin, neither is being an ass (the animal variety). However, a willful sinner is by his very nature out of communion with the Lord, unless he repents of his sinful ways.
As for Caiaphas, he was certainly not righteous in all his doings, but the passages detailing his prophecy link his prophecies to his office of High Priest. This position, prior to Christ’s ascension, entailed that he was the mediator between God and man, thus it was a role of his office (established and ordained by God) to prophecy. It’s my opinion that this alone allowed him to be both an unrighteous man, and to deliver prophecy.
Surely you're not implying that He is doing so again!