I have seen enough water damaged wood and cracked tile in old bathrooms to want to waterproof under the tub during my remodel.

After a lot of exercise, Schluter's Ditra looks like it will handle some movement from heating cables and waterproof the floor.

Schluter requires setting the tub apron onto the tile to distribute the weight over the Ditra. The only testing with Ditra was done with tile in place over it.

I have never seen a cast iron tub apron set on top of the tile.
Other than the challenges of installing the tub on a finished tile floor are there other reasons to bury the tub in the floor finish?

I am not planning heating cables under the tub so I have a 1/4" difference in levels between under the tub and outside the tub.
(If anyone has run cable under a cast iron tub to keep it warm I'm all ears.)

Can I get votes on setting the tub on tile 1/4" lower than the floor outside the tub and then set tile scribed to the tub over that tile?
Will you share what special prep and mortar are needed?