I have to agree that it's odd they say it can't be done. Any given ability must clearly have some kind of data flag in it which defines whether it does magical or physical damage, since that's necessary for it do actually do the correct damage type when playing. Why can't the client simply read this information and apply some sort of simple UI change to indicate an attack's damage type? I don't see why it needs to be some kind of "extra" thing added on to attack data that will somehow cause problems when they need to make last minute changes. Just let us read what's already there.

It doesn't need to be a fancy popup or anything, just changing the text colour of the flying text or cast bar is enough. Hell, even if all that happened was that the attack had (magical) or (physical) after its damage value in the battle log, this would still be miles more helpful than the current arrangement where it's literally a complete mystery, nothing in the game will tell you what type of damage you're receiving. Instead the only way to know if an attack is magical or physical is to either use third party addons (and the fact that these addons can read the properties of an attack and display its damage type should be more than enough evidence to prove that its possible in the actual game), or to simply run experiments to see if the damage is affected by magic-only or physical-only defense buffs.