They should just have Jon intermittently say "Isn't that right, Ghost?" and have a crew member growl a bit from off-camera.

Heh! Yes, that'd probably work better than having a dog that apparently no one can get to do what it's told. Can't explain Ghost's complete absence otherwise.

You'd think that for someone who was raised to marry her brother that would be the obvious solution. Problem would be to get goody-two-shoes Jon to go along with it.

Yes, going by Jaime and Cersei there does seem to be an incest taboo in Westeros. It just doesn't apply to Targaryans. But Jon hasn't been brought up as one.

I expect they'll find some reason to limit their use this season too.

Probably, or ice dragon or no, they ought to be able to deal with a big chunk of this undead army with relative ease.