We don't really know what the rules are if indeed there are any Obi-Wan may just be saying that to convince Luke to stay put (remember they are trying to prevent Luke from rushing in unready just to save his friends who don't actually need saving, they rescue him in the end and only just).
He may also be saying I can only protect you once and now is not the time.
When the Gods offer boons or magical weapons they often can only be used at special times and once used are lost.
If this was attempted it might mean having to adjust the ghost reunion at the end so Anakin appears first and then brings Ben and Yoda back.
The exorcism of Ben and Yoda by Palpatine could create anxiety in the audience (has Palpatine ended their afterlife?) my godson asked me to suggest that Yoda didn't die because it made him sad (he is nine), I suggested that it might necessary for the story and he said "Well he does come back as a ghost".
Putting that at risk and having Anakin being the agent of their return would add to his redemption.
It would have to be done with subtlety, Luke still has to be in pain in waves one and two, the ghosts should only appear where the lighting makes contact with them (think Captain Howdy in The Exorcist).
And as I said I'm not convinced it would work but it's worth a try (if anything to utterly dismiss the idea if it doesn't).