Yoda pretty much came about because Lucas needed a new Jedi master. If he ever considered ghost Ben training Luke, I’ve not read anything about it. Sir Alec was having health issues during production on ESB that put shooting his brief scenes in doubt for a while though.
Well didn’t Ben’s death come about because Alec Guiness didn’t want a big role and commitment to the rest of the franchise? In that case force ghost Ben as teacher would have defeated that purpose.